H. Ando

Recent progress in the multi-wafer CBE system

Recent progress in the multi-wafer CBE system has demonstrated the potential capability of producing the state-of-the-art compound semiconductor devices with excellent uniformity and reproducibility. The Al compositional uniformity of three 4 inch wafers grown at the same time is 0.2136 ± 0.0014 (standard deviation). The uniformity in thickness, composition, carrier concentration for three 3 inch wafers is within ±1.8%, ± 0.003 (Ga in InGaP), and ±2.2%, respectively. The temperature variation across a 3 inch GaAs wafer is ±0.9°C. The extreme abruptness of composition and the doping profiles using gaseous dopant sources are another remarkable advantage of this growth system. The extremely low surface defect density of 2.8 cm -2 for GaAs is routinely obtained. The InGaP/GaAs heterojunction bipolar transistor (HBT) with high current gain and superior high-frequency characteristics of excellent uniformity is successfully fabricated. These remarkable results are ascribed to the unique and superior design of the overall growth system : i.e. the novel metalorganic gas cells with the tilted aperture without any diffuser, the improved In-free substrate holder, the completely shut-off shutters, and the gas handling system.

Growth of carbon-doped base GaAs/AlGaAs HBT by gas-source MBE using TEG, TEA, TMG, AsH3, and Si2H6

High-performance carbon-doped-base GaAs/AlGaAs heterobipolar transistors (HBTs) were grown by gas-source MBE using only gaseous sources including dopant sources. The AlGaAs emitter layer was doped with Si from uncracked SI2H6 (n = 9 × 1017 cm-3), and the base layer (92.5 nm) was doped with carbon from TMG (p = 4 × 1019 cm-3). From SIMS analysis it was confirmed that a well-defined emitter-base junction with sharp carbon profile was obtained. The base-current ideality factor from the Gummel plot was 1.47, and the emitter-base junction ideality factor was 1.12. A high DC current gain of 53 was obtained at a current density of 4 × 104 A/cm2. The device characteristics of our carbon-doped HBTs were found to be stable under current stress.

Gas-source MBE growth of n-type InP using TEI, PH3, and Si2H6

This paper reports the results of a study in which disilane was shown to be an effective alternative gaseous dopant source in gas-source MBE growth of high-quality n-type InP using triethylindium and phosphine. The Si atomic concentration and the carrier concentration showed a linear dependence on the disilane flow rate over the range of carrier concentration investigated (7.8 × 1016-1.4 × 1018 cm-3). The electron mobilities and low-temperature photoluminescence properties indicate that silicon is mainly incorporated as a shallow donor impurity and the epilayers are low compensated.

Recent progress in multi-wafer CBE systems

Abstract We describe the recent progress of multi-wafer CBE systems demonstrating their potential capability of producing III–V semiconductor device structures with excellent uniformity and state-of-the-art performance. Up to three 4 inch wafers can be grown simultaneously with excellent uniformity and extremely small wafer-to-wafer variation (x in AlxGa1−x As: 0.2136 ± 0.0014). A specifically designed high-conductance metalorganic gas cell with a tilted aperture without any diffuser gives a high uniformity together with a fast switching at the heterointerfaces. An intrinsic difficulty for obtaining good compositional uniformity of ternary alloys containing indium is overcome by improvement in the design of an In-free substrate holder showing a uniform temperature profile within ± 0.9°C across a whole wafer. To extend the capability of CBE, an in situ cleaning method which combines hydrogen radical cleaning and As-free cleaning was investigated. The n-type GaAs epilayers showed a reduced depletion of electrons at the air-exposed regrown interface. From SIMS analysis, hydrogen radical and As-free cleaning show stronger effects on gettering carbon, and oxygen, respectively. Some C contamination during the 2nd As-free cleaning procedure is expected to be eliminated by decreasing the treatment temperature from the separately optimized one. The successful cleaning of InP substrates using trisdimethylaminophosphorous (TDMAP) would also extend the capability of CBE for the reproducible growth of large area (> 3 × 3 inch) InP-based materials.

High current gain InGaP/GaAs heterojunction bipolar transistors grown by multi-wafer gas-source molecular beam epitaxy system

Abstract We report for the first time on the high quality and highly uniform InGaP/GaAs heterojunction bipolar transistors with a carbon-doped base grown by a multi-wafer gas-source molecular beam epitaxy (GSMBE) system, which was developed by ourselves. A large-area uniform growth of p-type GaAs and InGaP has been realized. The variation in layer thickness of these layers was less than ± 1.8% across three 3-inch wafers. A large-area (200 × 200 μm 2 ) heterojunction bipolar transistor (HBT) with a base-sheet resistance of 350 Ω showed a high current gain of 190. This is effectively the highest value considering the base sheet resistance, and is comparable to the best data of AlGaAs/GaAs HBTs reported so far. The small signal current gains showed an excellent uniformity of 3% across one of five 2-inch wafers. The wafer-to-wafer variation of current gains for the two 2-inch wafers, in the same growth run, was less than 3%. The base sheet resistances also showed an excellent uniformity of 0.86%. In addition the average base sheet resistances of the two 2-inch wafers was the same. The device also showed an excellent high-frequency characteristics. The cut-off frequency ( f t ) of 61 GHz, and the maximum-oscillation frequency ( f max ) of 94 GHz were obtained for the transistor having a base dopant concentration of 4 × 10 19 cm −3 . These promising results demonstrate the high potential capability of multi-wafer GSMBE for the production of InGaP/GaAs HBTs.

GaAs/AlGaAs quantum well structures grown by gas-source MBE using TEG, TEA, and AsH3

Abstract We have studied the atomic steps and terraces at the interfaces of GaAs/AlGaAs single quantum well (SQW) structures grown by gas-source MBE on slightly misoriented GaAs substrates from (100) toward (111)A or (111)B plane with various misorientation angles α=0°, 0.5°, 1°, 2°, 4°, and 6°. The full-width-at-half-maximum of the photoluminescence (PL) peak from a gas-source MBE-grown QW having a well width of 1.4 nm on (100) flat substrates coincides with the calculated value, considering one-monolayer well-width fluctuation. We found that the 77 K PL linewidth of the QWs grown on (111)A-misoriented substrates decreases with increasing misorientation angle, and that, conversely, that of QWs grown on (111)B-misoriented substrates increases when the QWs were grown at a substrate temperature of 650°C. It was suggested that the heterointerface on (111)A-misoriented substrates consists of straight and regularly spaced steps, while that on (111)B-misoriented substrates consists of rough step edges, randomly spaced with step heights of several monolayers.

The electrical, optical and crystalline properties of GaAs: C grown by GSMBE using TMG and AsH3 for application to HBTs

Abstract We report the results of a comprehensive study on the electrical, optical and crystalline properties of heavily carbon doped p-type (100) GaAs epilayers ( p = 6.3 × 10 18 −1.3 × 10 20 cm -3 ; thickness = 250−420 nm) grown by gas source molecular beam epitaxy using trimethylgallium and arsine. X-ray analysis showed epilayer lattice contraction with a mismatch of δ a / a = -1.8 × 10 -3 at p = 1.3 × 10 20 cm -3 . Room temperature photoluminescence peak energy shifted from 1.40 eV ( p = 6.3 × 10 18 cm -3 ) to 1.37 eV ( p = 1.3 × 10 20 cm -3 ). Stokes Raman spectra showed two modes assigned as the unscreened LO phonon (292 cm -1 ) and the low frequency branch of the coupled hole-plasmon-LO-phonon (266 cm -1 ). Conservation of Raman scattering rules under all incident light configurations showed that the (100) GaAs:C epilayers were of high crystalline quality without the presence of faceting or other such crystalline defects. Annealing at 900°C for between 30 s to 45 min, resulted in a significant reduction in the hole concentration, lattice contraction and photoluminescence intensity for all epilayers. The implications of these results for the development of GaAs/AlGaAs HBTs are discussed.

Gas-source MBE growth and n-type doping of AlGaAs using TEG, TEA, AsH3 and Si2H6

We have studied gas-source molecular beam epitaxy (GSMBE) growth and n-type doping of AlGaAs using triethylgallium, triethylaluminum, arsine (AsH3) and disilane (Si2H6), focusing on (1) the effect of substrate temperature (520–690°C) and AsH3 flow rate (2–7 SCCM) on the carbon and oxygen incorporation of AlxGa1−xAs (x ∼ 0.28), and (2) the variation of the carrier concentration of n-type AlxGa1−xAs (x = 0–0.28) with Si2H6 flow rate (0.4–10 SCCM). The carbon concentration decreased with increasing substrate temperature up to 610°C, then increased with increasing substrate temperature using an AsH3 flow rate of 2 SCCM. Below 610°C, an increase in AsH3 flow rate resulted in a reduction in the carbon concentration. We obtained a carbon concentration of 1 × 1018 cm-3 at a substrate temperature of 520°C and an AsH3 flow rate of 7 SCCM. The addition of molecular hydrogen was found to further reduce the carbon concentration, and the lowest value obtained was 8.2 × 1017 cm-3 at a substrate temperature of 520°C using 4 SCCM AsH3 and 4.5 SCCM of molecular hydrogen. The oxygen concentration was not affected by the substrate temperature, but showed a slight decrease with increasing AsH3 flow rate. The lowest oxygen concentration was 2.5 × 1017 cm-3 at 7 SCCM AsH3 flow rate. The variation of the hole concentration with growth conditions was similar to that observed for carbon. The 4.2 K photoluminescence was dominated by a free-to-bound emission having a full-width-at-half-maximum of 18 meV, which is thought to be related to shallow carbon acceptors. Si2H6 was shown to be a suitable cold n-type gaseous dopant source for GSMBE growth of AlGaAs. The carrier concentration of the n-type AlxGa1−xAs (x = 0–0.28) epilayer was reproducibly controlled between 5 × 1017 and 2 × 1018 cm-3.

p+/N GaAs-AlGaAs heterostructures grown by gas source MBE using gaseous p- and n-type dopant sources

Abstract We report on the first growth of p + /N GaAs-Al x Ga 1− x As heterostructures using all gaseous sources by gas source MBE (GSMBE). GaAs was doped p-type (1.3×10 20 cm −3 ) using trimethylgallium as both a dopant and group III source and Al x Ga 1−x As ( x =0−0.28) was doped n-type (5×10 17 cm −3 ) using uncracked disilane as an n-type dopant and triethylgallium and triethylaluminium as group III sources. In both cases 100% arsine was used as the group V source. The current-voltage ideality factor of the heterojunctions was found to depend on the aluminium mole fraction of Al x Ga 1−x As ( x =0.12, 0.23, 0.28) and and the width ( Δs =5, 10, 20 nm) of an undoped GaAs spacer layer at the heterojunction. A value of 1.17 was obtained for a heterostructure with x = 0.23 and Δs = 20 nm .

Heavily carbon-doped p-type (In)GaAs grown by gas-source molecular beam epitaxy using diiodomethane

Abstract Heavily carbon-doped p-type In x Ga 1− x As (0≤ x 2 I 2 ), triethylindium (TEIn), triethylgallium (TEGa) and AsH 3 . Hole concentrations as high as 2.1 × 10 20 cm −3 were achieved in GaAs at an electrical activation efficiency of 100%. For In x Ga 1− x As, both the hole and the atomic carbon concentrations gradually decreased as the InAs mole fraction, x , increased from 0.41 to 0.49. Hole concentrations of 5.1 × 10 18 and 1.5 × 10 19 cm −3 for x = 0.49 and x = 0.41, respectively, were obtained by a preliminary experiment. After post-growth annealing (500°C, 5 min under As 4 pressure), the hole concentration increased to 6.2 × 10 18 cm −3 for x = 0.49, probably due to the activation of hydrogen-passivated carbon acceptors.