SiGe Photodetectors for Optical Telecommunications

MIDWAVE INFRARED DEVICES:

The band gap of antimonide alloys covers a wide spectral range from 300nm to 4.3um.  From a device point of view, the antimonide alloys are attractive for mid-infrared optoelectronic applications because they have a large split-off energy that leads to a reduced Auger recombination rate.   Despite this attractiveness optoelectronic devices grown on GaSb are difficult to realize because the mixed arsenide – antimonide heterostructures that are required for many devices have a type-II band alignment (see inset on MBE homepage).

The objective of this research is to design and epitaxially grow SiGe heterostructures for photodetectors at optical telecommunications wavelengths (the 1.3µm and 1.55µm channels) on Silicon substrates. To solve this problem we have designed a “W”-structure quantum well unit cell in SiGe that has a bandgap minimum sufficiently small to detect 1.3mm light.    Our approach relies on the type-II band alignment of Si and strained SiGe to create a bandgap minimum that is smaller than any of the constituent alloys.  A quick inspection of the calculated conduction and valence band edges reveals that the electron and hole do not reside in the same physical layers.  The design challenge wth this structure is to controllably engineer the quantum mechanical wave function of the electrons and holes such that there is a sizable overlap which leads to a strong interaction between the two, even though they “reside in different places”.

The structures that we have design are  called “W”-structures because the band edges appear to make a “W” profile. The SiGe structure that we designed consists of two 30A-thick Silicon electron wells surrounding a 17.5A thick Si0.61Ge0.39 hole well.  In this structure, the electron and hole wave functions concentrated in different regions, unlike traditional QW devices. Optical absorption is governed by the wave function overlap of the electron and heavy hole ground state wave functions. Single and multi-mode waveguide photodetectors have been fabricated and found to work.  A comparison of the spectral response of our SiGe photodetectors and that of a conventional silicon photodetector are also shown below. 





To create efficient devices using the type-II band alignment we have used designs from Dr. Jerry Meyer group at NRL for a “W” structure laser.  In a departure from other instances of this laser we have grown the devices of 0 alloys made from 8 elements (Al, Ga, In, As, and Sb with Si, Be, and Te dopants) using random alloy cladding layers instead of binary alloy layers.  Two key optimizations that were required for this structure to be realized was the calibration of a mixed arsenide-antimonide quaternary alloy, and the optimization of growth temperature of the active region.  With optimized growth conditions, the laser device shows strong peak around 3.5µm at low temperature and room temperature lasing with a threshold current density of 965 Acm-2.  The low temperature laser spectrum is shown in red.

METAMORPHIC GROWTH

There are often times when an alloy with an important bandgap cannot be realized on commonly available substrates such as GaAs, InP, or GaSb.  Other times, one may wish to mix material systems on a single platform.  Metamorphic growth describes the growth of a layer with a different lattice constant than that of the initial substrate. The lattice mismatches in the buffers are accommodated through the formation of dislocations. Key issues when growing metamorphic structures are to minimize the number of threading dislocations that penetrate through to the active region of the heterostructure, and to reduce the surface roughness caused by non-uniform growth on a strained surface.  We’ve demonstrated many metamorphic layers of antimonides on GaAs substrate, and InP grown on GaAs.