Apart from the relatively new GaAs-on-Si research, background information is presented at a minimum level. The main advantage of GaAs-on-Si technology is the special properties of Si substrates that are not available in GaAs substrates.
An Introduction
- The role of MBE in III-V technology
- Future trends in MBE
- An overview of activities in optical interconnects
- Signal fanout: electrical vs. optical interconnects
- Low-threshold lasers for inter-chip communication
- GaAs-on-Si detectors for clock synchronization
- References
Therefore, signals must be large enough to be interpreted by any element of the system. Assuming a tree fan-out structure, the total power loss can be written as where n is the number of fan-outs and T is the transmission percentage for each of the couplers in the tree.
Band Offsets at a Heterojunction
Green's function in semiconductors
The L-1 operator is defined as Green's operator for the L operator and is independent of the base choice. At this point we want to choose a basis and obtain the spectral representation of Green's function.
Physical significance of Green's function
This result was simplified by Garcia-Moliner and Rubio and Flores [2,3] if there is a planar interface, in which case we can write 2.13), then it will give the first of two matching conditions. In the following, we will use this result to examine existing theories and develop a new one for heterojunction band discontinuity prediction.
Quantum dipole theory
After this, the top 1.5 µm AlGaAs cladding layer was grown, followed by a p-GaAs capping layer doped with Be to 5.0 x 1019 cm -3 . The substrate temperature of the last growth of the GaAs layer abruptly dropped to 600°C. The doping level for both the p and n regions is lxl018cm-3. The structure of the p-i-n detector is shown in Figure 5.17.
A proposed model for band line-ups
Experimental data for theory testing
This implies that in a double Schottky barrier as illustrated in Figure 2.4(b), the band bending on either side of the Au layer is independent of the gold thickness and we can consequently think of the band offset ti.Ee as the value of 1 - 0 in the limit of zero gold thickness, for small mole fractions of aluminum x:s;0.3. This also confirms the predicted quasi-linear correlation by Tersoff [4] and Margritondo [22) for small aluminum mole fractions and indicates a relatively large difference for large aluminum mole fractions. Second, it may be similar to the history of the GaAs/AlAs system, which was first believed to have a near-zero valence band setting (85:15 rule) that was later changed, and therefore the current experimental data may not be reliable not. .
Conclusions
Even the recent Tersoff theory fails this test: it predicts a value of 0.35eV. More careful measurements will clarify this issue and establish the value of experimental band setup of InAs/GaSb system.
MBE Growth of GaAs-on-GaAs Quantum Well Lasers
Substrate preparation
First, when the growing condition is optimal, e.g. the substrate temperature for AlGaAs growth is 720°C, the two substrates produced almost identical surface morphologies favoring the tilted one slightly and they produce similar threshold current densities in wide-area lasers, with the tilted one slightly lower. As a result, using GaAs-on-Si saves only a small fraction of the total wafer cost after epitaxy. Current limitations of GaAs-on-Si are mainly due to the poor quality of the crystal growth.
7 A photograph of the first room-temperature, current-injected, continuous-wave (CW) operation in a large-area GaAs-on-Si laser. One of the most important goals in GaAs-on-Si research is to modulate GaAs lasers. The As cracker cell (manufactured by Perkin-Elmer) has two main parts: a large crucible in the back to hold enough As material to last a year, and a long and thin Mo tube in the front to crack the As4 into As2 • The Mo tube must always be heated before the source As gets hot to prevent As plugging the Mo tube.
Growth of GaAs/ AlGaAs quantum well lasers
Fabrication and measurement of broad area lasers
On the other hand, the impurity capture rate is relatively independent of the substrate orientation. In these cases, GaAs-on-Si is much more expensive due to the high cost of epilayer growth. After the success of CW operation, several other characteristics were measured to obtain a complete picture of the GaAs-on-Si laser.
The growth of the GaAs-on-Si p-i-n detector is the same as that of the lasers, except that the growth temperature was at 600°C for the entire p-i-n structure.
The effect of substrate orientations
Conclusions
The use of inclined substrates has resulted in the lowest threshold current density ever reported for any semiconductor laser. The effect of quantum well width on threshold current density is analyzed using both the k-selection rule and the non-k-selection rule. It turns out that in an undoped material the k-selection rule provides a very good fit to the experimental data.
Furthermore, k-selection rule gives a modal gain that is independent of Lz, compared to the non-k-selection result which shows an L-;1 dependence.
Potentialities and Limitations of GaAs-on-Si Research
- GaAs verses Si
- Advantages with GaAs-on-Si
- Limitations of GaAs-on-Si
- Current status of discrete devices and integrated circuits
- Conclusions
Difficulties arise in the processing of GaAs-on-Si devices when wafer cutting is required. A GaAs-on-Si laser will therefore have additional problems due to the splitting of two facets. Over the past few years, most forms of discrete microwave devices have been demonstrated in GaAs-on-Si.
GaAs-on-Si technology has already been used to fabricate most common microwave and optoelectronic devices, with some surprising successes.
MBE Growth of GaAs-on-Si Quantum Well Lasers
Special problems associated with GaAs-on-Si growth
When a polar semiconductor (GaAs) is grown on a non-polar semiconductor (Si), special problems arise that are not present in conventional polar-on-polar heteroepitaxy. Burgers vectors lie in the (100) plane and are parallel to (011) and (011) directions, and the type II dislocations, on the other hand, can move through the crystal by sliding along the (110) planes, since their Burgers vectors are inclined from the interface by 45° (Figure 5.2. Therefore, the objective is to increase the fraction of type I dislocations in the mismatched dislocation network and effectively reduce the fraction of type II active sources for generating screw dislocations .
Annealing experiments have been done by several groups and all of them report an improvement in the quality of the GaAs epitaxial layer [10].
Substrate preparation
After cleaning, the substrate is mounted on a home-made Mo-block for direct radiant heating. The mounted substrate is then loaded into the MBE system loading chamber, where the substrate is gradually heated to 900-1000°C for 2 minutes for oxide desorption. The substrate is then transferred to the growth chamber of the MBE system and the substrate is heated to approx. 900°C again to anneal the surface damage and blow off any impurity condensation on the surface during the transfer.
Finally, the substrate is cooled to an appropriate initial growth temperature to be discussed later.
Transition layer growth
To achieve both, the growth should be started at a low substrate temperature of about 200 °C and immediately after the first 1000 A of growth, the substrate temperature should be raised to 580 °C. When the substrate reaches 580 °C, the temperature of the Ga cell is set to what should give a growth rate of lµm/h. The thickness of the transition layer, which grows slowly and at low substrate temperature, is usually between 250-2500 A.
The use of a "cracker cell" can help solve this problem, since the initial growth temperature is above 400 °C and can be quickly increased to 580 °C, making the transition layer very thin and the voltage drop very small.
Room temperature CW operation of GaAs-on-Si lasers
Next, a very small ball of indium (they are commercially available from the Indium Corporation of America) is dropped into the flux and placed in the center of the diamond heat sink. In the case of a laser, the capacitance of the p-n junction can severely limit the modulation rate. This is used to monitor the temperature of the substrate surface before and during a growth.
Etch silicon dioxide in buffered hydrofluoric acid (OF HF) according to the thickness and etch rate of the BF HF.
Ridge-waveguide geometry lasers
As a result, GaAs is expected to outperform high-speed Si by a factor of 20 at the same sensitivity level. The width of the inner region varies from 1 to 3 µm to balance the transit time and the RC time constant. The output of the detector was then measured with both a sampling oscilloscope and a microwave spectrum analyzer.
For a 2µm intrinsic region, the impulse response shows a 45 ps FWHM, which corresponds to a 3dB bandwidth of 4 GHz (Figure 5.19).
Conclusions
This compares very well with results obtained with identical p-i-n GaAs-on-GaAs detectors fabricated using the same process [18]. Defect reduction has been shown to be a major technological barrier to the growth of high-quality GaAs-on-Si. Very satisfactory results were obtained using an As pre-layer and a low substrate temperature at the beginning of growth.
Record-breaking, low-threshold current-density lasers have been demonstrated, eventually leading to the first CW operation of room-temperature current-injected GaAs-on-Si lasers.
Operation and Maintenance of an MBE System
- Minimum system requirements
- The proper pumping procedures
- Handling and changing source materials
- Calibrations
- Conclusions
- References
Each part of an MBE system has a lifespan of, say, three years, but so many are active at the same time. Each time the MBE system has been opened for repair and/or charging it must be baked. As is often the case, we wait to open the MBE system to fix some broken parts while also replacing or refilling some source materials.
Si is available in ingot form and can be loaded into the MBE system without further purification.
GaAs wafer cleaning procedure
Technigue of growth interruption
7 Schematic drawing of the experimental setup used to measure (a) the threshold current and (b) the optical spectrum. However, many of our early experimental results suggest that the quantum well thickness should have very little effect on the threshold current density. Moreover, the non-k selection rule results in a modification of the transition probability [19,40] by a factor µenv.
Note that the equation does not depend on Lz and therefore the transparency current density for an ideal quantum well laser is independent of the well thickness. This illustrates the main advantage of quantum well lasers: a large reduction in the transparency current. The most striking feature of the no-k-selection rule is the linear increase of the threshold current density with the width of the quantum well.
However, for an epitaxially grown GaAs wafer, the cost of growth is much higher than for the substrate. In addition, the strip laser exhibited a much higher threshold current density than that of wide-area devices, and as a result, the heat release per unit area much higher.
Fabrication of broad area lasers
Si wafer cleaning procedure
Fabrication of stripe lasers
