A thin AlGaN cap is typically employed above the multiple quantum wells of InGaN-based lasers to prevent electron overflow from the active region and to protect the InGaN active region from the high temperature growth of subsequent p-type layers. The growth conditions and placement of this cap can significantly affect the efficiency and operating characteristics of laser diodes. A 200 Å Al0.2Ga0.8N:Mg cap was placed above the last barrier of a three quantum well (QW) laser diode, as well as directly above the last QW. Lasers with the cap above the last QW exhibit a lower threshold current and a higher internal quantum efficiency than lasers with the cap above the last barrier. The internal quantum efficiency nearly doubles from 16.6% for lasers with the cap above the last barrier to 34.7% for the cap above the last QW. The improvement in efficiency from moving the cap to the last QW is attributed to electrons falling into the last QW and participating in radiative recombination, instead of nonradiative recombination at dislocation sites of the last barrier, or recombining radiatively at the barrier wavelength, rather than at the laser emission wavelength.

The morphological evolution of InGaN laser diodes on laterally overgrown GaN on sapphire was investigated to understand the effect of threading dislocations (TDs) on the morphology and subsequent device performance. Screw-component TDs were found to strongly influence the surface morphology and growth mechanism of InGaN-based laser diodes. Spiral growth about mixed character TDs (Burgers vector=a+c) resulted in growth hillocks for laser structures grown directly on sapphire as well as laser structures grown on lateral epitaxially overgrown (LEO) GaN on sapphire. The dislocation distribution dominated the size of growth hillocks. Small spirals, with typical heights of 15Å and diameters of 1.5μm, were formed on laser structures grown on sapphire due to the uniformly distributed mixed character TDs, but large spiral hillocks, with heights of 65Å and diameters up to 30μm, developed on the LEO GaN due to the larger spacing between mixed character TDs. The spirals exhibited steps oriented in crystallographic directions, showing the six-fold symmetry of the hexagonal crystal. Spirals that formed during the growth of the active region did not diminish with subsequent high-temperature growth. Even with larger spirals, the lasers on LEO GaN exhibited reduced threshold current densities as compared to lasers grown on sapphire.

The carrier capture kinetics of the Ec—0.59 eV and Ec—0.91 eV electron traps found in molecular beam epitaxy (MBE)-grown n-GaN have been determined by means of deep level transient spectroscopy (DLTS). The 0.59 eV trap does not show the behaviour of either ideal point defects or line defects. In contrast, the 0.91 eV trap displays the kinetics of linearly arranged interacting point defects, which generate a time-dependent local Coulombic potential with a characteristic time constant of ≈ 8.6 μs.

Ultraviolet p-i-n photodiode structures have been fabricated using AlGaN/GaN and GaN structures grown on 6H-SiC. GaN photodiodes grown on 6H-SiC had lower leakage than GaN grown on sapphire. Solar-blind p-GaN/i-Al x Ga 1-x N./n-Al x Ga 1-x N (x 0.35) diodes were investigated. Leakage currents were measured to be as low as 10 nA/cm 2 at -5 V. Use of a thick insulating buffer resulted in a peak responsivity of 0.08 A/W at a wavelength of 292 nm, corresponding to an external quantum efficiency of 35%.

In this study, we use a quantum well (QW) probe structure to explore the size dependent effects of sidewall recombination in GaN. Mesas 0.8-7 μm in width with pitches of 4 μm, 8 μm, and 12 μm were etched into the QW probe structure, exposing the QW at the sidewalls. Several etch conditions were investigated. Room temperature photoluminescence (PL) measurements, using a He-Cd laser as an excitation source and laser spot size of approximately 230 μm, were taken before and after the mesas were etched. The effects of sidewall formation were quantified by comparing the maximum PL intensity of the QW before and after etch. Higher remaining PL intensity was observed for etch conditions which used both Ar ions and Cl 2 gas instead of only Ar ions. The fraction of remaining PL decreased with decreasing mesa width, however the remaining PL intensity was relatively large even for small features. The preliminary data suggested that GaN is relatively insensitive to sidewall damage.

We present the results of an approach that allows the analysis of grown-in defects in n-GaN grown by metal organic vapor phase epitaxy by relating the presence of various structural defects to the concentration and properties of distinct deep levels. Transmission electron microscopy and electron beam induced current (EBIC) microscopy together with post-growth hydrogenation are used to determine the threading dislocation density (TDD) and observe their local electrical activity. In conjunction, deep level optical and transient spectroscopies (DLOS and DLTS, respectively) are used to detect deep levels, determine their concentration and analyze their carrier trapping kinetics for these films. The comparison of EBIC analysis with trap spectra prior and after hydrogenation establishes a strong correlation between two specific levels, at E-c - E-t = 0.58 and 1.35 eV, and recombination centers distributed in the field of the GaN films. Further, EBIC analysis shows that, independent of hydrogenation, TDs behave as strong recombination centers, and indicates that there must be a deep level associated with these regions. A level observed at E-c - 2.64/E-v + 0.87 eV is a good candidate to account for the electrical activity of the TDs because it captures both electrons and holes, which is characteristic of recombination centers. This is supported by DLTS analysis of the trapping kinetics for this level that reveals a behavior characteristic of a linear arrangement of point defects likely found along the TDs.

Uniformly distributed precipitates have been observed by TEM in the p-type layers of laser structures. The precipitate density decreases with decreasing flow of biscyclopentadienyl-magnesium (Cp2Mg), which affects the hole concentrations in the p-type layers. The higher hole concentration, with the reduced precipitate density, reduces the threshold current density and improves the internal quantum efficiency because of the higher number of holes available for radiative recombination. The threshold current density is also reduced 30% from 20.8 V for lasers with a high precipitate density compared to 14.3 V for lasers with a lower precipitate density.

Although GaN is a chemically inert, thermally stable material, it has demonstrated sensitivity to ion damage generated by dry etch processes such as reacting ion etching and inductively coupled plasma etching. Recombination-enhanced diffusion is an important mechanism which has been observed in other III–V semiconductor systems. In this study we examine the possibility of enhanced diffusion in GaN using quantum well (QW) probe structures. The deeper QWs (750 and 1000 Å deep) showed a steady decrease in relative photoluminescence (PL) intensity with time, providing evidence of the cooperative effects of channeling and defect diffusion in deep etch damage propagation in GaN. In contrast, shallow QWs (150 and 250 Å from the surface) showed a slight decrease followed by a gradual increase in relative PL intensity with time which was explained by defect annihilation. Exposure to above band gap illumination, used to simulate and enhance carrier generation during etch, appears to speed defect annihilation in high defect concentration regions resulting in an increase in QW luminescence, where as in lower defect concentration areas, above band gap illumination does not appear to significantly alter QW luminescence. We attribute this difference in behavior to a difference in diffusion constant. The diffusion constant in less damaged regions may be much lower than that of the highly damaged material.

The influence of hydrostatic pressure on the emission and absorption spectra measured for various types of InGaN structures (epilayers, quantum wells, and quantum dots) is studied. While the known pressure coefficients of the GaN and InN band gaps are about 40 and 25 meV/GPa, respectively, the observed pressure-induced shifts in light emission energy in the InGaN alloys differ significantly from concentration-interpolated values. With increasing In concentration, and thus decreasing emission energy, the observed pressure coefficients become very small, reaching zero for emission energies similar to2 eV (roughly the value of the InN band gap). On the other hand, the pressure coefficient derived from absorption experiments exhibit a much smaller decrease with decreasing energy when referred to the same scale as the emission data. First-principles calculations of InGaN band structures and their modification with pressure are performed. The results are not able to explain the huge effect observed in the emission experiments, but they are in good agreement with the optical absorption data. Significant bowings of the band gap and its pressure coefficients are found, and they are especially large for small In concentrations. This behavior is related to the changes in the upper valence band states due to In alloying. Some possible mechanisms are discussed which might be expected to account for the low pressure coefficients of the light emission energy and the difference between the sensitivity of the emission and absorption to pressure.

The deep level spectra in both p+-n homojunction and n-type Schottky GaN diodes are studied by deep level transient spectroscopy (DLTS) in order to compare the role of the junction configuration on the defects found within the n-GaN layer. Both majority and minority carrier DLTS measurements are performed on the diodes allowing the observation of both electron and hole traps in n-GaN. An electron level at Ec−Et=0.58 and 0.62 V is observed in the p+-n and Schottky diodes, respectively, with a concentration of ∼3−4×1014 cm−3 and a capture cross section of ∼1−5×10−15 cm2. The similar Arrhenius behavior indicates that both emissions are related to the same defect. The shift in activation energy is correlated to the electric field enhanced-emission in the p+-n diode, where the junction barrier is much larger. The p+-n diode configuration allows the observation of a hole trap at Et−Ev=0.87 eV in the n-GaN which is very likely related to the yellow luminescence band.

The deep level spectrum of n-GaN grown by metal-organic chemical vapor deposition has been determined by means of a combination of deep level optical spectroscopy and deep level transient spectroscopy. Several deep levels are found across the bandgap at E-c-E-i=0.62, 1.35, 2.64-2.80 and 3.22 eV. The two deep levels found at 0.62 and 1.35 eV show strong H-passivation effects, with their concentrations decreasing by a factor of greater than or equal to 30 and similar to 14, respectively. However, electron beam induced current microscopy shows no changes in the electrical activity of the threading dislocations due to post-growth hydrogen incorporation, suggesting that point defects are responsible both for the 0.62 and 1.35 eV levels. The decrease in the 0.62 eV trap concentration together with correlation to the presence of Mg in n-GaN is consistent with Mg-H complex formation. The band of closely spaced levels observed at 2.64-2.80 eV narrows to 2.74-2.80 eV after hydrogenation, consistent with hydrogen complexing of V-Ga(3-) defects as anticipated by earlier theoretical results. The concentration of the 2.64-2.80 eV band of levels increases with electron irradiation consistent with generation of point defects, and supports the assignment of this band to V-Ga(3-) and related complexes. The 3.22 eV level is most likely related to background acceptors. Its concentration is not affected by hydrogenation, but it is highly dependent on electron irradiation, increasing with increasing electron flux.

N-Schottky and p+–n GaN junctions are currently used for different technologies. A comparison of the deep levels found throughout the entire band gap of n-GaN grown by metal-organic chemical vapor deposition under both configurations is presented. Both deep level optical spectroscopy and deep level transient spectroscopy measurements are used allowing the observation of both majority and minority carrier traps. Deep levels at Ec−Et=0.58–0.62, 1.35, 2.57–2.64, and 3.22 eV are observed for both diode configurations, with concentrations in the ∼1014–1016 cm−3 range. The 0.58–0.62 eV level appears correlated with residual Mg impurities in the n side of the p+–n diode measured by secondary-ion-mass spectroscopy, while the 1.35 eV level concentration increases by a factor of ∼4 for the Schottky junction possibly correlating with the carbon profile. The 2.57–2.64 eV level is a minority carrier hole trap in n-GaN, likely related to the yellow photoluminescence band, and is detected both optically from the conduction band (2.64 eV) and thermally from the valence band (0.87 eV).

Room temperature (RT) pulsed operation of blue nitride based multi-quantum well (MQW) laser diodes grown on c-plane sapphire substrates was achieved. Atmospheric pressure MOCVD was used to grow the active region of the device which consisted of a 10 pair In{sub 0.21}Ga{sub 0.79}N (2.5nm)/In{sub 0.07}Ga{sub 0.93}N (5nm) InGaN MQW. The threshold current density was reduced by a factor of 2 from 10 kA/cm{sup 2} for laser diodes grown on sapphire substrates to 4.8 kA/cm{sub 2} for laser diodes grown on lateral epitaxial overgrowth (LEO) GaN on sapphire. Lasing wavelengths as long as 425nm were obtained. LEDs with emission wavelengths as long as 500nm were obtained by increasing the Indium content. These results show that a reduction in nonradiative recombination from a reduced dislocation density leads to a higher internal quantum efficiency. Further research on GaN based laser diodes is needed to extend the wavelength to 490nm which is required for numerous bio-detection applications. The GaN blue lasers will be used to stimulate fluorescence in special dye molecules when the dyes are attached to specific molecules or microorganisms. Fluorescein is one commonly used dye molecule for chemical and biological warfare agent detection, and its optimal excitation wavelength is 490 nm. InGaN alloys can be used to reach this wavelength.

Etch damage of GaN was investigated using a quantum-well probe structure. A clear decrease in photoluminescence (PL) intensity was observed and was aggravated with increasing ion-beam voltage. The magnitude of decrease in PL intensity was much larger than expected, even greater than for GaAs subjected to similar etch conditions. Angle-dependent bombardment studies were carried out to investigate channeling as a damage mechanism in GaN. The large decrease in PL intensity observed near normal incidence or along the [0001] direction suggests that channeling is a damage mechanism for low-energy bombardment in GaN.

Unique constraints encountered in the III-N semiconductor system, such as a lack of cleaving planes and resistance to wet etchants, make conventional approaches to the fabrication of laser diodes difficult to implement, and result in the need for novel cavity designs. Improvements in in-plane cavities include etching facets using a focused ion beam and the incorporation of gratings to decrease losses associated with poor mirrors. Towards the fabrication of an electrically pumped VCSEL, the issue of the bottom mirror can be addressed by the use of dielectric mirrors, and either the removal of the sapphire substrate or its incorporation into the cavity by using a curved backside mirror. This paper will review recent developments in these areas at UC Santa Barbara.

We have studied an influence of pressure on the emission and absorption spectra measured from various types of InGaN structures such as epilayers, quantum wells and quantum dots. While the known pressure coefficients of GaN and InN bandgaps are in the range 40-25 meV/GPa, the experimental observation for the light emission shift with pressure for InGaN alloys is dramatically different. With the increasing In content and thus decreasing emission energy the observed pressure coefficients become very small eventually reaching zero or even slightly negative values! We have observed a much weaker trend for the decrease of the pressure coefficient for the absorption edges of InGaN. First principle calculations of InGaN band structure and its modification with a pressure are not able to explain the huge effect observed in the emission experiment but are in a good agreement with the results obtained in optical absorption measurements. We discuss here the possible mechanisms which can account for extremely low pressure coefficient of the light emission and the discrepancy between sensitivity light emission and absorption on applied pressure in InGaN alloys.

Differential postgrowth hydrogen passivation of deep levels in n–GaN grown by metal-organic chemical vapor deposition has been directly observed by means of both deep level transient spectroscopy and deep level optical spectroscopy. Two deep levels found at Ec−Et=0.62 and 1.35 eV show strong H passivation effects, with their concentrations decreasing by a factor of ⩾30 and ∼14, respectively. The decrease in the 0.62 eV trap concentration together with its correlation with the presence of Mg in n–GaN is consistent with Mg–H complex formation. A band of closely spaced deep levels observed at Ec−Et=2.64–2.80 eV narrows to Ec−Et=2.74–2.80 eV after hydrogenation, consistent with hydrogen complexing with VGa3− defects as anticipated by earlier theoretical results. Finally, a deep level at Ec−Et=3.22 eV likely related to background acceptors remains unaffected by hydrogen.