IEEE ELECTRON DEVICE LETTERS. VOL. EDL-7, NO. zyxwvutsrqpon 2, PEBRUARY 1986 zyxwvutsr 129 Gao,72A10.28As/Gao 99Beo zyx olAs Heterojunction Bipolar Transistor Grown by Molecular Beam Epitaxy J. L. LIEVIN, C. DUBON-CHEVALLIER, F. ALEXANDRE, G. LEROUX, J. DANGLA, AND D. ANKH Abstract-We report the first heterojunction bipolar transistor (HBT) with base doping level as high as 2 zyxwvutsrqp x 1020 ~m-~. The device is grown by molecular beam epitaxy (MBE) with growth conditions adjusted zyxwvuts fj keep perfect surface morphology and to atoid dopant diffusion even at ultra- high doping levels. Maximum dc current gain of 10 is observed, for abase thickness of 40 nm. This isa new step in the optimization of HBT's structures for high-speed logic and microwave applications. A I. INTRODUCTION lxGal-,As/GaAs heterojunction bipolar transistors (HBT's) grown by molecular beam epitaxy (MBE) have recently demonstrated high-speed capabilities [l], [2] and appear promising for high-frequency and logic applications. For a bipolar transistor, the speed performances can be strongly improved by reducing the electron transit time through the device, as well as the base resistance-collector capacitance product. Thanks to a wide gap emitter, an HBT can exhibit a very high current gain even at base doping levels higher than the emitter one [3]. We present in this paper an optimized expitaxial structure for high-speed HBT using a thin ultra-heavy doped base layer. Recently,. p+ + type doping levels into GaAs up to 2 x zyxwv 1020 cm-3 have been demonstrated [4] while maintaining a good layer morphology compatible with multilayer growth for device applications. At high-base doping levels, the electron lifetime in the base is affected, while the injection efficiency zyxwvu y can be kept close to unity thanks to a graded A10.28G~,72A~- GaAs interface. As a consequence, the current gain p of an HBT should be directly related to the base transfer factor. However, on the contrary of these theoretical predictions, y was found to be below unity even for very high current-gain phototransistors grown by MBE [3]. In particular, the diffu- sion of Be in the (Ga,Al)As emitter layer can reduce y, as observed by electroluminescence zyxwvuts [5]. Then, the current gain p of the HBT becomes a critical parameter when using ultra- heavy doped base layers. We report in this paper the first epitaxial growth of an HBT structure with base doping level as high as 2 x lozo ~m-~. The base layer material becomes a Ga,,99Beo,olAs alloy, as will be discussed. The device is not notably affected by Be diffusion, and exhibits dc current gain up to 10. Manuscript received October 14, 1985; revised November 26, 1985. The authors are with the Centre National d'Etudes des TBlBcommunica- tions, Laboratoire de Bagneux, 196 rue de Paris, 92220 Bagneux, France. IEEE Log Number 8407394. 11. EXPERIMENTAL PROCEDURE Epitaxial layers were grown in a Riber 2300 UHV system on (001)-oriented Cr-doped GaAs substrates. The crystal quality was checked during the growth by observation of streaky reconstructed RHEED (10 keV) patterns. Typical substrate temperatures, measured by a calibrated optical pyrometer, were 600, 450, and 620°C during growth of the collector, base, and emitter regions, respectively. A cross section of the device is shown in Fig. 1. The semi-insulating substrate had a 700-nm GaAs buffer layer followed by a 450- nm-thick collector doped to zyxw n = 5 X 10l6 ~m-~. The p t + type 2 X lozo base was 40-nm thick and separated from the emitter by a 10-nm undoped GaAs spacer. Next, a 300-nm- thick G Q , , ~ A ~ ~ , ~ ~ A S emitter doped to 5 X IO" ~ m - ~ was grown, with graded AI mole fraction down to about zy x = 0.05. Finally, a 200-nm-thick GaAs cap layer doped to 2 X cm-3 was deposited to facilitate formation of ohmic contacts. The role of the undoped GaAs spacer layer is to partially offset the diffusion of Be into the emitter. The presence of such spacer layers has been shown experimentally to lead to significant current gain increases [6]. The layers were processed with a simple double mesa technology. The first mesa is performed with an isotropic, controlled etch rate solution (H3PO4-H2O2-Hz0) to contact the base. Since the base layer is very thin, the etching is stopped at about 50 nm from the interface. The base layer will be contacted by the diffusion of the ohmic contact dopant impurity. A second etching permits to contact the n + buffer layer. The isolation is then performed using proton and and boron implantation. The emitter-collector ohmic contact is classically AuGeNi. For the base ohmic contact, AuMn is evaporated [7]. The penetration of Mn in GaAs during the alloying cycle (800"Cimin raising up to 400 "C) is around 100 nm. The specific contact resistivity calculated using the transmission line method was in this case 4 X Q*cm2. 111. RESULTS AND DrscussroN Reducing the substrate temperature below the usual MBE growth conditions appears to be anefficient way to incorporate more active Beryllium impurities into GaAs, as well as to minimize the dopant diffusion [3]. We have checked by Secondary Ion Mass Spectroscopy (SIMS) anaysis the Bery- llium and Aluminium distributions in the epilayers of a similar HBT structure (except for base thickness: zyxw W, = 80 nm). The 0741-3106/86/0200-0129$01.00 0 1986 IEEE