GaInP Selective Area Epitaxy for Heterojunction Bipolar Transistor Applications
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n-GaAs Collector
9 in electronic devices such as HBTs.
15 Here, a novel HBT structure that incorporates multiple epitaxy to selectively regrow the emitter is demonstrated. This HBT seen in
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Figure 1. Schematic of proposed regrown emitter HBT. 253
Mat. Res. Soc. Symp. Proc. Vol. 448 01997 Materials Research Society
figure 1 has a number of advantages over a conventionally fabricated device:
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The wide bandgap extrinsic base produces a larger turn-on than the intrinsic base, suppressing emitter edge current. This passivation scheme can be significantly smaller in dimension than the depleted emitter ledge. The use of GaLnP instead of AIGaAs precludes the problems with DX center defects16-18 as well as facilitating the process through the use of wet etchants selective between GaInP and GaAs. In addition, it has been shown that regrowth with phosphorus containing materials has superior compositional uniformity than arsenic
compounds. 19-22
The three layer extrinsic base stack gives a lower overall base resistance, leading to highfMAx with low base transit time. The emitter dimension is reduced from the photolithographic stripe size with Si0 2 sidewall spacers, similar to Si bipolar technology.23,24
To successfully fabricate this device, a significant amount of regrowth technology must be demonstrated. First, the selective growth of high quality Ga.521n.4 8P lattice matched to GaAs in the presence an SiO 2 dielectric mask must be demonstrated. Then it is necessary to investigate the forward biased IV characteristics of the critical regrown base-emitter junction. As a final measure of the SAE quality, complete devices should be fabricated and measured. EXPERIMENT All sample growths were done on [001] S.I. GaAs substrates. The base and collector layers of the diode and device samples prior to regrowth were grown by MBE. The collector consists of a 6000A n+ GaAs buffer Si doped at -5x10 1 8 cm-3, a lower collector layer 3000A thick doped to 3x1017 cm-3, and an upper collector layer 3000k thick doped to 3x 1016 cm-3. The base layers are 19 composed of a three layer stack: 500A p+ GaAs intrinsic base layer Be doped to Wx10 cm500k p+ GaInP wide bandgap extrinsic base layer Be doped to lx 1019 cm-3 , and 1500A p+ GaAs base contact layer Be doped to lxlO9 cm-3 . Approximately 3500k of SiO 2 was deposited over these by PECVD at 300'C. The SiO 2 was patterned by photolithography with either a wet buffered HF etch solution or an anisotropic dry etch using CHF 3 plasma. Test diodes for regrowth evaluation had the simplified structure shown in figure 2. The top p+GaAs layer was etched off prior to SiO 2 deposition. After etching the SiO 2 pattern, the exposed GaInP was selectively wet etched in an HCI:H 3PO 4 :H20 solution. Prior to regrowth samples were solvent degreased then dipped in a dilute NH 4OH:H 20 surface etch. A detailed description of the complete HBT device process has been discussed previously.25 Selective area regrowth was accomplished by low pressure (20 Torr) metal-organic vapor phase epit
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