ALGAAS MQW SURFACE-NORMAL MODULATORS PART 1

As described in the previous three chapters, optical interconnects may replace electrical interconnects for short distance communication links.  In order for this vision to become  a  reality,  optoelectronic  transmitter  devices  must  be  developed  that  are compatible with both the electrical and optical systems.  Several requirements for these optoelectronic transmitters were laid out in Chapter 3.   This dissertation covers two surface-normal modulators designed for this application.  This chapter aims to describe our first optoelectronic modulator, designed, fabricated and tested in the AlGaAs/GaAs semiconductor material system [1].

BASICS

As reviewed earlier, semiconductor optoelectronic modulators can be fabricated in a surface-normal geometry in order to allow integration of 2D arrays of devices with Si CMOS circuitry.  The electronic signal is applied as a voltage across two pads at the top of the CMOS chip, which is directly connected to a p-i-n diode modulator.  The CMOS voltage reverse biases the diode and changes the electric field across the MQW region. The modulating electric field causes a change in the absorption of the MQWs according to the QCSE, as outlined in the previous chapter.  Thus, a CW laser beam incident on the modulator will have its intensity altered according to the state of the voltage across the CMOS pads

DESIGN

The optical interconnects group at Bell Labs (Lucent Technologies) in the mid-1990’s succeeded in designing and fabricating 2D arrays of AlGaAs-based modulators for precisely this purpose [2-4].  Using the Bell Labs modulator as a starting point, we developed our own wafer design and growth, fabrication process, and hybrid integration technique using indium-based flip-chip bonding.

Description	Material	Thickness (Å)	Dopant
p-cap	p-GaAs	100	[Be]=1x1019
p-layer	p-Al0.3Ga0.7As	2030	[Be]=1x1019
i-(MQW) layer 50x	GaAs AlGaAs	95 30	----
n-layer	n-Al0.3Ga0.7As	5000	[Si]=4.4x1018
buffer layer	GaAs	500	----
etchstop layer	Al0.85Ga0.15As	2800	----
substrate	GaAs	~500 µm	----

Fig. 4.2. Wafer design for AlGaAs modulators

This section describes the wafer structure choices that must be made in order to implement a p-i-n diode modulator.  The first layer grown (i.e. closest to the substrate) had to be an AlxGa1-xAs etchstop layer.   During the final processing steps, the GaAs substrate is removed by wet chemical etching.   This requires removing ~500 µm  of GaAs, stopping on the final 1 µm uniformly across a chip that is 1.2 mm wide or larger. The high Al concentration (x ≥ 85%) of this layer enables a highly selective wet etch to remove layers of GaAs at an etch rate significantly greater (about 100-1000 times faster) than the etch rate for the AlGaAs etchstop [5].  The next layer, a GaAs buffer, is used during the cavity tuning step described in section 4.7.  The n-AlGaAs layer is used with an ohmic contact to apply an electric field across the MQW region.  The quantum well design was dictated by the requirement that the modulators operate at a wavelength close to 850 nm, where the lasers used in the optical-interconnect-testing setup emitted light. Finally, the p-AlGaAs and p-GaAs cap were grown to enable p-type ohmic contact.