Atomic layer etching (ALE) was performed on (Al, In, Ga)N thin films using a cyclic process of alternating Cl2 gas absorption and Ar+ ion bombardment in an inductively coupled plasma etcher system. The etch damage was characterized by comparing photoluminescence of blue single quantum well light-emitting diodes before and after the etch as well as bulk resistivities of etched p-doped layers. It was found that etched surfaces were smooth and highly conformal, retaining the step-terrace features of the as-grown surface, thus realizing ALE. Longer exposures to the dry etching increased the bulk resistivity of etched surfaces layers slightly, with a damaged depth of ∼55 nm. With further optimization and damage recovery, ALE is a promising candidate for controlled etching with atomic accuracy. It was found that Al0.1Ga0.9N acts as an etch barrier for the ALE etch, making it a suitable etch to reveal buried V-defects in III-nitride light emitting diodes.

Semiconductor devices often require band-gap engineering, which in turn requires the use of alloys and the tuning of their composition to achieve the required electrical or optical behavior. While their compositional fluctuations are generally treated as a perturbation in most conventional III-V semiconductors, the effects of alloy disorder are much more significant in the III-nitrides. Here, the effects of alloy disorder on carrier localization are compared for different III-V semiconductors, particularly for holes for which localization effects are more significant due to their heavier effective mass. This study is conducted using three-dimensional computation for III-V alloys with natural (random) compositional fluctuations. Given the complexity of the problem, we carry out the computations relying on a simplified Hamiltonian in the envelope wave-function approximation with a single heavy-hole valence band. We investigate the effects of compositional fluctuations on carrier localization using two methods. The first method, based on the localization landscape theory, is used to solve the localization landscape equations and obtain the effective potentials acting on carriers. This potential acts as a confining potential that predicts the regions of spatial localization of carriers, and thus allows a comparison of the effects of alloy fluctuations between the conventional III-V and the III-nitride semiconductors. We find that the effective potential of the III-nitride semiconductors exhibits much larger fluctuations compared to the other III-V semiconductors. This might point to a higher degree of carrier localization in the nitrides, particularly for holes. This is verified through the second method, solving Schrodinger's equation and obtaining the electron and hole wave functions. We find that for InxGa1-xN the electron wave functions are delocalized even for the ground state, whereas the low-energy hole states are localized. This is in contrast with the behavior of holes in the common alloy InxGa1-xAs, which are found to be always delocalized. Thus, our study shows the importance of accounting for alloy disorder in the nitrides.

While GaN has been researched for many years as a material for light emitting diodes, lasers, high electron mobility transistors, and other devices, further improvements to material purity are still needed to realize the full potential of this material for vertical device applications. To achieve high quality, high purity GaN for power devices, our research employs ammonia-assisted molecular beam epitaxy (NH 3 -MBE) and plasma-assisted molecular beam epitaxy (PA-MBE). Past studies of fast growth rates (more than 1 μm/hr) and ultra-low impurity incorporation revealed that both PAMBE and NH 3 -MBE can achieve low net doping levels, and NH 3 -MBE as low as 1x10 15 cm -3 . 1 However, This work also indicated that impurity incorporation increased with increasing growth rate. Recent research efforts have focused on further improvements to morphology and unintentional dopants in NH 3 -MBE-grown homoepitaxial layers with fast growth rates. By optimizing growth conditions for faster growth rates and using indium as a surfactant, devices with drift layers up to 10 μm thick were grown. These layers exhibited smooth surface morphology as seen in AFM scans with RMS surface roughness as low as 0.21 nm for a 2x2 μm 2 scan area. 2 Adding the In surfactant also led to reduced incorporation of unintentional Si impurities in UID films as shown by SIMS depth profiles for GaN films grown with and without In. The extremely low impurity levels were further confirmed by CV measurements which indicated that the net doping density in the UID GaN layers decreased from 2x10 16 cm -3 to 5x10 15 cm -3 with the addition of the In surfactant. 2 Based on these UID levels and the theoretical critical electric field of GaN (3 MV/cm), drift layers up to 29.5 μm and 7.4 μm can be depleted with a triangular electric field profile for net donor concentrations of 5x10 15 cm -3 and 2x10 16 cm -3 , respectively. Devices with the same net concentrations with drift regions 3 μm thick can hold voltages of 854 V and 717 V, respectively. Based on the previously described studies of low-impurity GaN at different growth rates, we developed vertical p-n GaN power devices grown by NH 3 MBE. 3 These p-n diodes contained a 0.25 µm n+ region, 4 µm n- region with 3x10 15 cm -3 unintentional carriers, 0.4 µm p+ region and 10nm p++ capping layer to improve the ohmic anode contact. The circular diodes were fabricated with Pd/Pt anode contacts and Ti/Au cathode contacts. Device diameters were between 80 and 100 µm. The devices featured a 15 µm field plate made from Ti/Au contacts on top of an Al 2 O 3 /Si 3 N 4 dielectric stack. The diodes exhibited outstanding forward J-V behavior with a low specific on-resistance (R on,sp ) of 0.28 mΩ-cm 2 and a minimum ideality factor of 1.36, which are among the best achieved among similar homoepitaxial GaN p-n diodes. 3-5 The best diodes also exhibited a breakdown voltage of > 1 kV (equipment limit 1 kV). The reverse-voltage properties indicate that punch-through was achieved with a peak electric field (E C ) >2.6 MV/cm, as confirmed by Silvaco simulations (Fig. 7). The combination of >1 kV breakdown voltage and on-resistance of 0.28 mΩ-cm 2 represents one of the best performances of the p-n GaN diodes, achieved with a significantly thinner drift layer compared to comparable MOCVD GaN p-n diodes with drift layers more than 8 µm. Work is ongoing to develop NH 3 -MBE p-n diodes with indium surfactant-assisted drift layer growth which shows promise to further improve the surface morphology, reduce leakage, and enhance breakdown performance of the vertical devices.

We demonstrate vertical integration of nitride-based blue/green micro-light-emitting diodes (µLEDs) stacks with independent junctions control using hybrid tunnel junction (TJ). The hybrid TJ was gown by metal organic chemical vapor deposition (p + GaN) and molecular-beam epitaxy (n + GaN). Uniform blue, green and blue/green emission can be generated from different junction diodes. The peak external quantum efficiency (EQE) of the TJ blue µLEDs and green µLEDs with indium tin oxide contact is 30% and 12%, respectively. The carrier transportation between different junction diodes was discussed. This work suggests a promising approach for vertical µLEDs integration to enhance the output power of single LEDs chip and monolithic µLEDs with different emission colors with independent junction control.

GaN has achieved widespread interest in high-power and high-speed electronics due to its wide-bandgap (WBG), large breakdown field, and high mobility compared to Si and SiC. There is also continued interest in the growth and development of GaN vertical devices owing to their advantageous properties of high current capability from backside ohmic, better field management, scaling feasibility, and enhanced thermal transport, which is not feasible with the lateral device topology. To realize the predicted high breakdown voltage and low on-resistance from vertical devices, high-quality drift layers with low unintentional doping and low compensating defects are fundamentally required. Hence, our research aims to pursue a comprehensive investigation of growth and device development with vertical GaN structures, building from the optimization of thick GaN homoepitaxy with high growth rate but low impurities, improving surface morphology using Indium as a surfactant as well as realizing punch-through field-plate p-n diodes to maximize the average electric field in high-voltage operation. Toward the goal of high-quality GaN epitaxy growth, we performed systematic studies of growth rate effects on the drift layer doping using ammonia molecular beam epitaxy (NH 3 MBE) and plasma-assisted molecular beam epitaxy (PA MBE) [1]. The NH 3 MBE homoepitaxy GaN demonstrated an unintentional doping (UID) of ~10 15 cm -3 , significantly lower than that of the conventional metalorganic chemical vapor deposition (MOCVD) reports, which can be potentially contributed by clean growth environment of MBE. We further focused on improving surface morphology using Indium as a surfactant. Through a combination of varying V/III ratios, In flux and growth temperatures, an optimal condition for surface morphology, characterized by atomic force microscopy, was achieved for fast growth rates (on the order of 1 µm/hr and beam equivalent pressures on the order of 5 × 10 -7 Torr). However, excessive indium causes the surface morphology to degrade, potentially due to the enhancement of the Ga desorption from the surface as a result of the reaction of indium with ammonia for high indium fluxes. The indium surfactant can also reduce silicon impurities as revealed from the Secondary ion mass spectrometry (SIMS) (Fig. 2). Subsequently, we developed vertical p-n GaN power devices using the NH 3 MBE-grown high-quality epitaxy [2]. The vertical p-n GaN diode consisted (from bottom up): ~0.25 µm of n+ GaN ([Si]: 1×10 19 cm -3 ) buffer, ~4 µm UID GaN drift layer for n-type region, ~0.4 µm p+GaN ([Mg]: 3×10 19 cm -3 ) for p-type region, and ~10 nm p++ GaN cap ([Mg]: 3×10 20 cm -3 ) to support the anode ohmic contact. Circular p-n diodes of 80-100 µm diameter were fabricated with Pd/Pt anode and a backside Ti/ Au ohmic contact. The field management was achieved by field-plate of 15 µm formed with Ti/Au metals on top of a field-plate dielectric of a Al 2 O 3 (26 nm)/ Si 3 N 4 (205 nm) stack (Fig. 3). The p-n GaN diodes exhibited an outstanding forward J-V behavior with a low specific on-resistance (R on,sp ) of 0.28 mΩ-cm 2 and a minimum ideality factor of 1.36 (Fig. 4), which are among the best achieved in vertical homoepitaxy GaN p-n diodes [2-4]. The best diodes also exhibited a breakdown voltage of > 1 kV (equipment limit 1 kV). These reverse-voltage properties imply a punch-through characteristics with peak electric field (E C ) >2.6 MV/cm, as revealed by Silvaco simulation (Fig. 5). The combination of >1 kV breakdown voltage and on-resistance of 0.28 mΩ-cm 2 represents one of the best performances of the p-n GaN diodes, achieved with a significantly downscaled (~4 um) drift layer compared to the MOCVD GaN p-n diodes of ≥ 8 um (Fig. 6). Work is ongoing to develop NHE MBE p-n diodes with indium surfactant-assisted drift layer growth which will be promising to further improve the surface morphology, reduce leakage, and enhance breakdown performance of the vertical devices. References: [1] J. Wang, K. Jorgensen et al. APL Mater. 9 , 081118 (2021) [2] E. Farzana et al, IEEE Electron Device Lett . 41 (12) , 1806 (2020). [3] H. Fu et al, IEEE Electron Device Lett . , 41 , 127, (2020). [4] Z. Hu et al," Appl. Phys. Lett. , 107 , 2435011, (2015). Figure 1

We report unipolar hole transport through unintentionally doped (UID) c-plane Ga-polar InGaN heterostructures to investigate the impact of alloy disorder on vertical transport. Simulations and experimental investigations were conducted on unipolar InGaN double heterostructures (DHs) and quantum well (QW) test structures by varying thicknesses and the number of QWs. The structures were simulated using one-dimensional (1D) and three-dimensional (3D) algorithms, incorporating the effects of random alloy disorder in the 3D model using the newly developed Localization Landscape theory. The electrical polarization discontinuity between GaN and InGaN results in a significant barrier for vertical carrier transport. Band-diagram and current density-voltage simulations indicate asymmetric polarization barriers to the vertical hole transport for the InGaN DHs. For the QW structures, however, the simulations indicate a symmetric barrier to the hole transport in forward and reverse bias. In the case of the InGaN DH layers, the 3D simulation results indicate a smaller barrier to hole transport compared to 1D simulations. For QW simulations, the barriers were found to be the same in both 1D and 3D simulations. The simulation results are experimentally verified using unipolar p-type vertical transport structures, enabled by n-to-p tunnel junctions to facilitate the current spreading within the bulk material for the mesa structures grown by ammonia-molecular beam epitaxy. The results indicate that increasing the UID In0.1Ga0.9N DH layer thickness from 15 to 30 nm increases the forward bias voltage drop (similar to 2 Vat 500 A/cm(2)) more than the reverse bias voltage drop (similar to 0.2 V at 500 A/cm(2)). For the QW structures, increasing the number of QWs from one to three increases the voltage penalty similarly in forward and reverse directions (similar to 0.25 V per QW at 500 A/cm(2)). The results are beneficial in understanding the impact of alloy disorder on the transport properties of the III-nitride heterostructures.

We report on experimental and simulation-based results using (In, Ga)N alloy quantum barriers in c-plane green light-emitting diode (LED) structures as a means to improve vertical carrier transport and reduce forward voltage (VF). Three-dimensional device simulations that include random alloy fluctuations are used to understand carrier behavior in a disordered potential. The simulated current density???voltage (J-V) characteristics and modified electron-hole overlap |Fmod|2 indicate that increasing the indium fraction in the (In, Ga)N quantum barriers leads to a reduced polarization discontinuity at the interface between the quantum barrier and quantum well, thereby reducing VF and improving |Fmod|2. Maps of electron and hole current through the device show a relatively homogenous distribution in the XY plane for structures using GaN quantum barriers; in contrast, preferential pathways for vertical transport are identified in structures with (In, Ga)N barriers as regions of high and low current. A positive correlation between hole (electron) current in the p-side (n-side) barrier and indium fraction reveals that preferential pathways exist in regions of high indium content. Furthermore, a negative correlation between the strain ??zz and indium fraction shows that high indium content regions have reduced strain-induced piezoelectric polarization in the Z direction due to the mechanical constraint of the surrounding lower indium content regions. Experimentally, multiple quantum well green LEDs with (In, Ga)N quantum barriers exhibit lower VF and blue-shifted wavelengths relative to LEDs with GaN quantum barriers, consistent with simulation data. These results can be used to inform heterostructure design of low VF, long-wavelength LEDs and provide important insight into the nature of carrier transport in III-nitride alloy materials.

We report on the improvement of the surface morphology of c-plane GaN films grown at high growth rates (∼1 µm/h) using ammonia molecular beam epitaxy through a series of growth optimizations as well as the introduction of indium as a surfactant. The indium surfactant was expected to help with the adatom mobility and, thus, provide smoother growth surfaces. Through a combination of varying V/III ratios, In flux, and growth temperatures, an optimal condition for surface morphology, characterized by atomic force microscopy, was achieved. At higher Ga fluxes for fast growth rates (∼1 µm/h and beam equivalent pressures of ∼5 × 10−7 Torr), higher ammonia flows were necessary to preserve the surface morphology. In addition, indium was an effective surfactant—reducing the roughness and improving the overall surface morphology. However, excessive indium causes the surface morphology to degrade, potentially due to the enhancement of the Ga desorption from the surface as a result of the reaction of indium with ammonia for high indium fluxes. The indium surfactant also resulted in a reduction of background Si impurity concentrations in the film. These effects allow for the growth of thick drift layers with low background dopant concentrations for vertical GaN power devices.

We report on ammonia and plasma-assisted molecular beam epitaxy (NH3-MBE and PAMBE) grown GaN layers with a low net carrier concentration (Nnet). Growth parameters, such as growth rate, V–III ratio, and plasma power, were investigated on different substrates to study their impact on surface morphology and background doping levels using atomic force microscopy and capacitance–voltage (C–V) measurements, respectively. The elevated growth rates are especially interesting for vertical power switches, requiring very thick drift regions (over 10 μm) with low background concentrations. For our NH3-MBE-grown layers, Nnet shows an almost linear increase with the growth rate. Using a freestanding substrate and at a fast growth rate of 1.4 μm hr−1, a Nnet value as low as 1 × 1015 cm−3 was achieved. For samples grown via PAMBE, the lowest Nnet among samples grown under a Ga adlayer was 2 × 1016 cm−3 for a growth rate of 0.32 μm h−1 on a GaN-on-sapphire template. The results support the use of MBE for growing high-quality GaN material with reasonably fast growth rates maintaining low background doping levels for high-voltage vertical power electronic devices.

We demonstrate efficient cascaded blue/green micro-size light-emitting diodes (μLEDs) with independent junction control. The cascaded μLEDs, consisted of blue μLEDs, a tunnel junction, and green μLEDs, were fully grown by metalorganic chemical vapor deposition. Blue, green, and blue/green emissions can be independently controlled in the same device. The blue μLEDs (60 × 60 μm2) and green μLEDs (40 × 40 μm2) exhibit a forward voltage of 4.1 and 3.1 V at 20 A/cm2 and a high peak external quantum efficiency of 42% and 14%, respectively. This demonstration paves the way for monolithic integration of full color cascaded μLEDs.

Finite element analysis software was used to model and visualize two p-n junction models: one with a single threading dislocation (TD) and a control without one. TDs are modeled as a Gaussian distribution of trap states with a full width at half maximum value of 5 nm localized around the r=0 line in a cylindrical coordination such that the linear trap state density was 1 trap c−1-translation; this model allows the cylindrical symmetry of the c-plane GaN crystal orientation to be used to avoid more computationally intensive 3D models. In this work, a vertical p–n diode with typical doping characteristics and an equivalent threading dislocation density of 108 cm−2 was modeled in reverse bias. Our simulations show that the dislocation-mediated leakage mechanism for reverse bias leakage in GaN p–n diodes is the generation of electron–hole pairs via a trap-assisted tunneling mechanism whereby electrons from the valence band use the intermediate trap state to traverse the band gap. This mechanism results in electron–hole pairs that are swept out of the junction by the reverse bias electric field. This behavior results in a measurable leakage current within the model with behavior consistent with experimental values.

In this work, we present fully transparent metal organic chemical vapor deposition (MOCVD)-grown InGaN cascaded micro-light-emitting diodes (µLEDs) with independent junction control. The cascaded µLEDs consisted of a blue emitting diode, a tunnel junction (TJ), a green emitting diode, and a TJ, without using any conductive oxide layer. We can control the injection of carriers into blue, green, and blue/green junctions in the same device independently, which show high optical and electrical performance. The forward voltage (Vf) at 20 A/cm2 for the TJ blue µLEDs and TJ green µLEDs is 4.06 and 3.13 V, respectively. These results demonstrate the efficient TJs and fully activated p-type GaN in the cascaded µLEDs. Such demonstration shows the important application of TJs for the integration of µLEDs with multiple color emissions.

We experimentally characterize mid-infrared intersubband transitions in identical Al 0.5 Ga 0.5 N/GaN heterostructures grown on a- and m-plane GaN substrates. The absorption peaks of the m-plane samples are 10 to 40% narrower than that of the a-plane samples.

We report on the vertical hole transport through unipolar unintentionally doped (UID) and p-type doped AlGaN heterostructures to evaluate the effectiveness of the UID and doped AlGaN as barriers to the hole transport. Band diagram and current density–voltage (J–V) simulations are conducted in one-dimensional and three-dimensional schemes, with the latter including compositional fluctuations within the alloy AlGaN barrier layer. The simulation results using a self-consistent Poisson-drift diffusion scheme, incorporating the Localization Landscape theory, indicate a large asymmetric barrier to the hole transport by UID AlGaN. The asymmetric J–V characteristics are attributed to the asymmetric band diagrams calculated for the unipolar structure. The simulation results are verified by experiments using unipolar vertical hole transport structures enabled by n-to-p tunnel junctions (TJs) grown by ammonia molecular-beam epitaxy. The TJ structures are utilized to minimize the issues with the high spreading resistance of p-regions and to eliminate the need for its dry etching, which normally results in degraded p-contacts. The experimental results show that even a thin UID AlxGa1−xN (x = 14%, 13 nm) introduces an asymmetric barrier to the hole transport; a nearly 100% increase in the voltage drop induced by a thin UID AlGaN at 50 A/cm2 in the reverse direction is observed compared to an only 25% corresponding increase in the forward direction. Furthermore, p-type doping of the AlGaN layer results in a drastic drop in the potential barrier to hole transport in both directions. The results are beneficial for understanding the behavior of various structure designs within optoelectronics and power electronics.

Mid-infrared intersubband transitions are investigated in nonpolar m-plane and a-plane GaN/AlGaN multi-quantum well heterostructures. Nominally identical heterostructures were grown by ammonia molecular-beam epitaxy on free-standing m-plane and a-plane GaN substrates. A total of 12 well- and barrier-doped samples with intersubband transition energies in the range of 220–320 meV (wavelength range 3.8–5.6 μm) were grown. The intersubband absorption lines of the m-plane samples were 10–40% narrower than those of the a-plane samples, and a very narrow intersubband absorption linewidth of 38 meV (full width at half maximum) at a transition energy of approximately 250 meV (5 μm wavelength) was observed in an m-plane sample. Narrower intersubband absorption linewidths of m-plane samples can be explained by more abrupt heterostructure interfaces revealed by structural characterization, which is attributed to a higher stability of the m-plane compared to the a-plane. No significant difference in the intersubband absorption linewidth was observed between the barrier- and well-doped samples.

Vertical GaN-on-GaN p-n diodes grown by ammonia molecular beam epitaxy (NH 3 MBE) are reported in this letter for prospective high-power application devices. The diodes, consisting of only 4 μm drift layer with low unintentionalbackgrounddoping of ~ 3×10 15 cm -3 , showed a maximum breakdown voltage of >1.0 kV and a low differential specific on-resistance of 0.28 mΩ-cm 2 . Moreover, these diodes exhibited an excellent rectifying behavior with a low minimum ideality factor of 1.36 and a punch-through electric field comparable to the state-of-the-art vertical GaN-on-GaN p-n diodes grown by metalorganic chemical vapor deposition (MOCVD). These results suggest that all-MBE vertical GaN-on-GaN p-n diodes grown by NH 3 MBE can be promising for small-size high-power electronic devices.

Finite element analysis software was used to model and visualize two p-n junction models: one with a single threading dislocation (TD) and a control model without a dislocation. TDs are modeled as a Gaussian distribution of trap states with an FWHM of 5 nm localized around the r=0 line in a cylindrical coordination such that the linear trap state density was 1 trap/c-translation; this model allows the cylindrical symmetry of the c-plane GaN crystal orientation to be utilized to avoid more computationally intensive 3D models. It was discovered that the interaction of the charged dislocation region with the p-n junction had many notable effects. At zero bias, it was observed that the depletion region width (using the Depletion Approximation) and the maximum electric field were markedly reduced near the dislocation line. More significantly, an asymmetric reduction in the diffusion barrier for electrons (Vbieff=3.03eV) and holes (Vbieff=0.81eV) was observed due to the asymmetric nature of the dislocation band bending related to the doping. The asymmetric reductions in diffusion barriers persisted into VA=2.4V leakage case where asymmetric current profiles for electrons and holes were also observed. Lastly, the diffusion barrier reduction resulted in an additional Shockley-Read-Hall nonradiative recombination leakage caused by a high np-product and trap state density near the intersection of the dislocation with the junction.