T. F. Kuech

Mechanism of carbon incorporation in MOCVD GaAs

Abstract The incorporation of carbon in GaAs epitaxial layers, an important problem in MOCVD, has been studied over a wide range of experimental parameters: growth temperature, AsH 3 Ga(CH 3 ) 3 ratio, carrier gas, as well as substrate orientation. The carbon incorporation, monitored by photoluminescence (2 K), is reduced both at low growth temperatures and high AsH 3 TMG ratios. The substrate surface orientation strongly influences the rate of carbon incorporation with layers on the 〈111〉 As surface exhibiting the highest carbon concentration and layers on 〈111〉 Ga the lowest. These results support a model of carbon incorporation in which reactive hydrocarbons are adsorbed on surface arsenic atoms.

Determination of the interdiffusion of Al and Ga in undoped (Al,Ga)As/GaAs quantum wells

We have employed photoluminescence spectroscopy to determine the temperature dependence of the interdiffusion coefficient of Al and Ga in GaAs/Al0.3Ga0.7As quantum wells. The position of the photoluminescence peaks, due to the n=1 electron to heavy‐hole transition, was measured before and after annealing the samples. A variational calculation was employed to determine the expected position of these photoluminescence peaks and from this a value of the interdiffusion coefficient was extracted. The interdiffusion process is characterized by an activation energy of about 6 eV leading to an interdiffusion coefficient at 850 °C of 4×10−19 cm2/s. This technique allows for the measurement of small diffusion coefficients in a wide variety of material systems.

Nonalloyed ohmic contacts to n‐GaAs by solid‐phase epitaxy of Ge

A low resistance nonalloyed ohmic contact to n‐GaAs is formed which utilizes the solid‐phase epitaxy of Ge through PdGe. Discussion focuses on the conditions necessary to attain low specific contact resistivity (∼10−6 Ω cm2 on 1018 cm−3 n‐GaAs) and on the interfacial morphology between the contact metallization and the GaAs substrate. MeV Rutherford backscattering spectrometry and channeling show the predominant reaction to be that of Pd with amorphous Ge to form PdGe followed by the solid‐phase transport and epitaxial growth of Ge on 〈100〉 GaAs. Cross‐sectional transmission electron microscopy and lattice imaging show a very limited initial Pd‐GaAs reaction and a final interface which is planar and structurally abrupt to within atomic dimensions. The presence of excess Ge over that necessary for PdGe formation and the placement of Pd initially in contact with GaAs are required to result in the lowest contact resistivity. The experimental data suggest a replacement mechanism in which an n+‐GaAs surface re...

Controlled carbon doping of GaAs by metalorganic vapor phase epitaxy

The controlled incorporation of carbon has been demonstrated for the metalorganic vapor phase epitaxy of GaAs. Carbon levels between 1016 and 1019 cm−3 can be achieved under typical growth conditions by using Ga(CH3)3 and either As(CH3)3 or mixtures of As(CH3)3 and AsH3. The carbon incorporation into GaAs goes through a minimum with growth temperature at ∼650 °C when using Ga(CH3)3 and As(CH3)3. The controlled addition of AsH3 monotonically decreases the carbon incorporation. The high carbon levels (≳1–2×1019 cm−3), greater than the reported solid solubility, are thermally stable with a low diffusion coefficient. The GaAs:C layers exhibit a low deep level concentration, ∼1013 cm−3, with only a single midgap trap present.

Properties of high‐purity AlxGa1−xAs grown by the metalorganic vapor‐phase‐epitaxy technique using methyl precursors

The metalorganic vapor‐phase epitaxy (MOVPE) of AlxGa1−xAs most commonly employs the methyl precursors Al(CH3)3 and Ga(CH3)3. These precursors were used in the growth of AlxGa1−xAs over the entire range of alloy composition in a low‐pressure horizontal MOVPE reactor. A complete chemical, electrical, and optical characterization of high‐purity MOVPE AlxGa1−xAs grown over the entire range of growth temperatures (600–800 °C) was carried out in order to determine the relationship of the materials properties to the growth conditions. Carbon, the primary impurity in the layers, dominates the electrical properties of the epitaxial layers. A superlinear dependence of carbon incorporation on AlAs mole fraction is observed, along with a two‐slope dependence on growth temperature. Photoluminescence spectra (2 K) were obtained from materials with AlAs mole fraction over the range 0≤x≤0.80. The photoluminescence intensity of the layers also exhibits a systematic dependence on alloy composition and growth temperature. ...

The influence of growth chemistry on the MOVPE growth of GaAs and AlxGa1−xAs layers and heterostructures

Abstract A comparison study was carried out on the influence of the growth chemistry on the properties of Al x Ga 1− x As and GaAs layers and quantum well structures. Triethylgallium, triethylaluminum, trimethylgallium, and trimethylaluminum were used in a various combinations during MOVPE growth of Al x Ga 1− x As. Substantial reductions in the carbon incorporation can be achieved using the ethyl based growth chemistry. The observed change in the carbon incorporation with growth chemistry indicates a change in the decomposition kinetics and mechanisms between the various growth precursors. While triethylgallium can be directly substituted for trimethylgallium in the growth of Al x Ga 1− x As, the use of triethyl aluminum requires particular care. Narrow quantum well structures were demonstrated using both ethyl and methyl based precursors.

p-GaN surface treatments for metal contacts

The chemical bonding and electronic properties of wet, chemically treated p-GaN surfaces were studied using synchrotron radiation photoemission spectroscopy. Chlorine-based chemical bonding was identified on the conventional HCl-treated p-GaN surface, which is associated with a shift of the surface Fermi level toward the conduction band edge by ∼0.9 eV with respect to the thermally cleaned surface. Compared to the HCl-treated surface, the surface Fermi level on the KOH-treated surface lies about ∼1.0 eV closer to the valence band edge, resulting in a much smaller surface barrier height to p-type materials than the HCl-treated surface. The smaller surface barrier height to p-GaN after KOH treatment can lead to a lower contact resistivity and can play an important role in lowering the metal contact resistivity to p-GaN.

High temperature adduct formation of trimethylgallium and ammonia

High temperature gas phase reactions between trimethylgallium (TMG) and ammonia were studied by means of in situ mass spectroscopy in an isothermal flow tube reactor. The temperature, pressure, and reaction time were chosen to emulate the gas phase environment typical of the metal–organic vapor phase epitaxy (MOVPE) of GaN. The main gas phase species is [(CH 3)2Ga:NH2]x, where most probably x=3, resulting from the very fast adduct formation followed by elimination of methane. The further gas phase decomposition of this species proceeds through the stepwise elimination of methane. These studies indicate that little TMG exists within the growth ambient under most MOVPE growth conditions. The further gas phase reaction of [(CH3)2Ga:NH 2]x may be responsible for the strong dependence of the MOVPE GaN growth rate and uniformity commonly observed.

Pressure dependence of GaAs/AlxGa1−xAs quantum‐well bound states: The determination of valence‐band offsets

We report experiment and theory on the pressure dependence of quantum‐well bound states formed in the GaAs/AlxGa1−xAs heterostructure system. Using MQW’s and SL’s of various barrier compositions x, we trace in photoluminescence (8 K), and in full‐scale pseudopotential calculation, the pressure‐induced evolution of the lowest spatially confined states within the wells. With increasing pressure Γ‐confined states follow the shift to higher energies of the direct GaAs band gap. At critical pressures a crossing occurs between these Γ bound states and the barrier indirect X states. Here, Γ intensities plunge and new emission tracking the X edges appears. Confirmed in wave function calculation, these new transitions occur across the heterointerface, between X‐confined electrons within the AlxGa1−xAs and Γ‐confined holes within the GaAs. Arising from valence‐offset‐induced staggered band alignment, critical pressures for observation of these states decrease with increasing Al mole fraction. We thus obtain, with p...

Application of selective epitaxy to fabrication of nanometer scale wire and dot structures

The selective growth of nanometer scale GaAs wire and dot structures using metalorganic vapor phase epitaxy is demonstrated. Spectrally resolved cathodoluminescence images as well as spectra from single dots and wires are presented. A blue shifting of the GaAs peak is observed as the size scale of the wires and dots decreases.