Extended abstract of a paper presented at MC 2007, 33rd DGE Conference in Saarbrücken, Germany, September 2 – September 7, 2007

Extended abstract of a paper presented at MC 2007, 33rd DGE Conference in Saarbrücken, Germany, September 2 – September 7, 2007

Extended abstract of a paper presented at MC 2007, 33rd DGE Conference in Saarbrücken, Germany, September 2 – September 7, 2007

Extended abstract of a paper presented at MC 2007, 33rd DGE Conference in Saarbrücken, Germany, September 2 – September 7, 2007

Extended abstract of a paper presented at MC 2007, 33rd DGE Conference in Saarbrücken, Germany, September 2 – September 7, 2007

The visualization of fine topographic details of the cell surface using “wet-mode imaging” is very often difficult. At high magnification and under standard conditions ( temperature=+3 °C, pressure= 5.0 TORR, WD= 7-9 mm, V= 10-15 kV) it is not possible to identify fine surface structures like microvilli and mini blebs (micron and sub-micron range). These structures seem to be masked by a fogy like layer. In addition to this, a peculiar contrast effect with alternating dark and light areas that not correspond with the topography of the cell, can be observed (Figure 1). This phenomenon disappear after drying of the specimen and do not reappear completely after rehydration.

This paper reports on the epitaxial growth of GaAs by means of molecular beams of Ga(CH3)3 (TMG) or Ga(C2H5)3 (TEG) and AsH3. Using TMG as the Ga source the films are p-type with carrier concentrations between 3×1019 and 3×1020 cm-3 caused by carbon incorporation. Substituting TMG by TEG shifts the kinetically limited growth region to lower temperatures. Moreover, the background doping is reduced by several ordes of magnitude (into the 1014–1015 cm-3 range) and even weakly n-type layers can be obtained. This behaviour is explained by the different Ga-C bond strengths in the metalorganic compounds.

We report on the intentional ρ-type doping of GaAs layers grown in an UHV system from molecular beams of arsine (AsH3) and mixtures of frimethyl gallium (TMG) and friethyl gallium (TEG). The entire doping range between 1014 cm-3 (growth from pure TEG) and 1020 cm-3 (growth from pure TMG) can be covered by using mixtures of TMG and TEG. As revealed by SIMS and photoluminescence (PL) carbon is the dominant acceptor in the layers. Comparison of the Hall mobility and of the PL spectra shows that the quality of our films equals that of the best LPE and MBE grown ρ-type GaAs layers.

Ultraviolet stimulated growth of GaAs from AsH3 and Ga(CH3)3 or Ga(C2H5)3 was studied using 254 nm irradiation, which dissociates all three reactants. Growth from the ethyl compound takes place at a temperature ∼150° lower than that using the methyl compound, indicating the important role of the alkyl in the rate-determining step. The temperatures at which UV irradiation affects the rate show a similar difference. The radiation effects on rate and morphology are more pronounced for the methyl than for the ethyl compound at the temperatures investigated (783–923, and 653 K, respectively). Some important implications of the data for the kinetics of the process are discussed.

This paper describes a comprehensive study of selective epitaxial deposition of GaAs on partly masked GaAs substrates using TMG or TEG and AsH3 in H2 (MOCVD) at 5X102 to 105 Pa total pressure, and in an ultra high vacuum apparatus without carrier gas (MOMBE). A minimum temperature is required to obtain localized growth. This temperature increases with the overall pressure in the system and is larger for TEG than for TMG. It is proposed that the presence of an adsorbed As species on the SiO2 is responsible for the breakdown of selectivity.

The role of the molecular hydrogen in the MOCVD of GaAs using Ga(CH3)3 (TMG) and AsH3 (arsine) was studied by comparing hydrogen and the inert element helium as carrier gases. It was found that the growth rate in He is systematically about 10% lower than in H2 over the entire range of conditions investigated. The background doping level is mainly determinated by the arsine partial pressure but not by the selection of the carrier gas. It is argued that the rather subordinate role of H2 suggests that the formation of GaAs takes place by direct interaction between TMG and AsH3.

The effect of total system pressure on the uniformity of epitaxial GaAs layers grown at 1023 K in the LP-VPE system has been studied with respect to layer thickness and dopant uptake. It was found that the uniformity of both of these quantities is distinctly improved when the hydrogen pressure is lowered from 3.3×104 to 6.5×l03 Pa. This behaviour is explained by the increase in gas phase diffusivity and the reduction in depletion of the gas phase during GaAs growth at reduced total pressure.

In this paper we report on the growth of GaAs in a UHV system from molecular beams of trimethyl gallium (TMG) and arsine (AsH3). Deposition could only be achieved if the AsH3 was partially decomposed before injecting it into the system. If arsenic is provided in excess, the growth rate depends linearly on the TMG beam pressure. The growth rate saturates the system if the arsenic pressure in the beam is reduced; the saturation value being proportional to the arsenic flux. The results can be explained by a model assuming that sticking of TMG at the growing surface is allowed only if As simultaneously is supplied in excess. The layers are heavily doped with carbon ( p≊1019–1020 cm−3). The carbon uptake is reduced by several orders of magnitude if TEG is used rather than TMG.

We have studied the growth of GaAs films from Ga(CH3)3 and AsH3 in a H2 carrier in the pressure range from 104 to 105 Pa. Over this entire range one finds a temperature region where growth is mainly limited by the supply of reactants. It is bounded at the high temperature side by a region of reduced growth caused by depletion of the gas phase due to precipitaton at the walls. At the low temperature side the rate drops off again, here due to surface kinetic limitations. The uniformity of film thickness improves upon reduction of the pressure. However, the increasing importance of surface reactions in the overall growth process has a negative effect on the morphology of the layers and limits the usable range of deposition parameters.

The effect of UV irradiation on the growth of GaAs from Ga(CH3)3 and AsH3 has been studied for different experimental conditions. As expected, in regions where surface kinetics plays a role in determining the rate a distinct influence of the light was found. In all cases the morphology is improved and for many conditions an increase in rate was obtained. A model is proposed in which the dissociation of TMG in the gas phase and the resulting hydrocarbon fragments play an important role.

In this study we discuss the application of an earlier proposed model to GaAs growth in the HCl-Ga-AsH3-H2 system at hydrogen pressures in the range 103-105 Pa. The model takes into account the kinetic steps of gas phase diffusion, adsorption of the various species from the gas phase on the crystal surface and the chemical reaction itself, which produces the material to be deposited. A good quantitative description of the dependence of growth rate on hydrogen pressure, growth temperature, and input pressures of HCl and AsH3 is obtained. The model leads to a better understanding of the growth mechanism and allows proper choice of the deposition parameters in order to obtain optimum material characteristics.

A study was made o f the doping o f e p i t a x i a l GaAs f i lms w i t h Te and S i n the HC1-Ga-AsH3-Hz system a t reduced pressures us ing Te added t o the Ga source o r H2S i n j e c t e d i n the deposi t ion reg ion. For both dopants reduct i o n of the Hz pressure leads t o increased dopant concentrat ions i n the f i l m s . The use of gas phase concentrat ions l a r g e r than those y i e l d i n g the maximum a t t a i n a b l e e lec t ron concentrat ions (3x1018 cm-3 f o r S, 6xl018cm-3 f o r Te) leads t o reduced values for the e lec t ron m o b i l i t y . It i s concluded from t h i s behavior t h a t the uptake o f dopants cont inues i n neu t ra l form beyond these concentrat ions. A q u a l i t a t i v e model descr ib ing the data on dopant incorporat i o n i s presented. I. Introduct ion. Vapor phase epi taxy o f 111-V compounds a t low pressures i s o f cons ide rab le i n t e r e s t due t o i t s a b i l i t y t o deposi t f i l m s w i t h e x c e l l e n t p roper t ies /1,2/. For the growth o f h igh q u a l i t y GaAs layers the low pressure technique has been app l ied t o the metalorganic Ga(CH3)3-AsH3-H2 system (LP-MOCVD) /3/ and t o the i n o r ganic systems HC1-Ga-AsH3-H2/4/ and AsC13-GaAs-Hz /5/ (LP-VPE). I n order t o o b t a i n f i l m s w i t h optimum homogeneity both growth r a t e and i m p u r i t y uptake have t o be constant over the e n t i r e wafer area. Because o f t h e increased d i f f u s i v i t y of t h e gaseous species a t lower pressures a more homogenous gas phase compos i t i o n over the subst rate may indeed be expected, leading t o f i l m s w i t h improved u n i f o r m i t y of thickness and dopant concentrat ion compared t o those grown a t atmospher i c pressure. However, besides the e f f e c t of t o t a l pressure, the p a r t i a l pressures of the var ious gaseous species and the growth temperature a l s o a f f e c t the condi t ions o f growth and impur i t y uptake a t the subst rate surface. I n an e a r l i e r study the growth process i n a low pressure system has been inves t iga ted ex tens ive ly /4/. A b e t t e r understanding o f the p a r t i c i p a t i n g k i n e t i c processes cou ld be obtained, a l lowi n g t o propose optimum experimental condi t ions. For s i m i l a r reasons the goal o f t h i s i n v e s t i g a t i o n i s t o ob ta in more i n s i g h t i n t o the process o f i m p u r i t y uptake. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1982513 C5-102 JOURNAL DE PHYSIQUE This study has been carried out in t h e HC1-Ga-ASH -Hz system. Since the growth ra te may'be varied over a wide range (from approximate31y 40pm/h to values below lpm/h) by only changing the hydrogen pressure (from 1 bar to 0.01 bar) a t the same part ial pressures of the other components /4/, t h i s system i s very suitable for the growth of multilayer structures with films of different thicknesses. As doping elements we have chosen sulfur and tellurium. Sulfur has a relat ively high diffusivity in GaAs /6/, but i t was chosen because of i t s easy handling in the form of H2S. Also, the large amount of data from experiments at' atmospheric pressure, available in the l i t e rature /1,7-lo/, makes a comparison interesting. Te was investigated because of i t s low diffusion coefficient in GaAs /6/; we have used the method of doping from the Ga source, which we have successfully applied in an ea r l i e r study fo r t i n and zinc doping /11/. In addition to the experimental data on both dopants we will present a quali tat ive model describing the incorporation of these elements. 2. Experimental .The problems, arising fo r growth a t reduced hydrogen pressures in the inorganic chlorine transport system and the resulting requirements on reactor design, were discussed in ref . 4. Fig. 1 shows a schematic of the reactor, used f o r

Deposition of from the system at 1023 K and pressures from 1 down to 0.01 bar leads to controlled growth rates ranging from 40 to 1 μm/hr. Due to the surface kinetic limitation of the process, very uniform films are obtained at reduced pressures. The background impurity level is low; 77 K electron mobilities of 73,000 cm2 V−1 sec−1 were obtained for not purposely doped samples with electron concentrations in the upper 1014 cm−3region. Using as a dopant, the electron concentration increases linearly with the pressure. The sulfur‐doped films show the same values for the mobility and the saturation concentration of electrons as layers obtained at atmospheric pressure.