Al x Ga1 − x As    ( 111 )  A  Substrate with Atomically Flat Polished Surface

Abstract

The design of lattice matching between substrate and epitaxial layers is of critical importance in semiconductor device fabrication. The need for alloy semiconductor substrates is recognized, but their crystal growth is rather difficult. Therefore present device design uses binary compound semiconductor substrates, such as GaAs, InP, etc. If alloy semiconductor substrates can be utilized, new device applications will develop. For this reason, we have grown AlGaAs bulk single crystals by float zone (FZ) using a controlled vapor pressure method (FZ-CVP). 1 One of us (J.N.) has pointed out the importance of controlled vapor pressure in various kinds of compound semiconductors crystal growth. 2-4 FZ-CVP can regulate stoichiometry by controlling vapor pressure precisely and can grow high purity crystals due to the absence of a container. Moreover this method can control segregation along the growth direction by zone traveling. It is found that Al x Ga 1Ϫx As (x < 0.05) substrate grown by FZ-CVP and the active layer in light emitting diode structures grown on this AlGaAs substrate have extremely low deep level density, a factor of 8-2000 less, compared with GaAs substrates grown by the horizontal Bridgeman method. 1 The deep levels were detected by the photocapacitance method Nishizawa devised. 5 The diameter of AlGaAs crystals grown by FZ-CVP has increased up to 20 mm by adopting novel dual frequency induction heating. 1 Recently, the A-face (Ga-rich face) such as (111)A and other (n11)A substrates have been tried in optical devices and 2D electron devices. [7][8][9][10] The main reason for this is obtaining a flatter interface. But there are peculiar difficulties in GaAs A-face polishing. The (111)A GaAs polishing surfaces at present are less smooth than the (100) surfaces. Moreover (n11) substrates are not available commercially.In this paper surface roughness values [root-mean-square (rms) value] of commercially available GaAs substrates (100) and (111)A measured by phase-measuring laser interferometoric microscope are shown first. This phase-measuring laser interferometoric microscope is inferior to atomic force microscope (AFM) with respect to horizontal resolution by two orders of magnitude. But vertical resolution is good enough for nanometer-scale measurement.Then atomically flat polished surfaces of Al x Ga 1Ϫx As (x ϭ 0-0.1) substrate grown by FZ-CVP are discussed. The polishing solution used is composed of H 2 O, NaClO, (CH 3 CO) 2 O, and KOH. This polishing solution is called spA hereafter. Sodium hypochlorite (NaClO) solution is known to be useful for GaAs wafer polishing. 11 The NaClO solution needs to be more dilute for GaAs (111)A polishing than for polishing of other faces. However, in doped GaAs (111)A, striations appear when diluted NaClO solution is used. Also in the case of AlGaAs (111)A, extensive striations appear when using dilut-