Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure – With heterojunction
Reexamination Certificate
1999-01-14
2001-03-13
Jackson, Jr., Jerome (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
With heterojunction
C257S094000, C257S096000, C372S050121
Reexamination Certificate
active
06201264
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to the field of III-V semiconductor devices and more particularly to the fabrication of devices with aluminum (Al)-containing III-V semiconductor materials.
BACKGROUND ART
The general background of the present invention is disclosed in U.S. Pat. No. 5,262,360 by Holonyak, Jr. and Dallesasse (Holonyak '360) entitled “AlGaAs Native Oxide” granted Nov. 16, 1993 and U.S. Pat. No. 5,517,039 by Holonyak, Jr. et al (Holonyak '039) entitled “Semiconductor Devices Fabricated with Passivated High Aluminum Content III-V Materials” granted May 14, 1996. Holonyak '039 is co-assigned with the present invention to the Hewlett-Packard Company.
High aluminum (Al) containing III-V semiconductor materials degrade in wet; high-temperature environments due to the formation of an undesirable Al-oxide, primarily thought to be Al[OH]
3
. These oxides tend to be optically absorbing and limit the transmission of light from light emitting semiconductor devices. The poor Al-oxide can also attack the crystal structure of the device.
One method of preventing device degradation is to grow a high quality native oxide that effectively seals the device and prevents the formation of a poor, optically absorbing Al-oxide. Native oxides are formed at higher temperatures and include Al(O)OH and Al
2
O
3
. A device is considered passivated if the native oxide prevents or significantly reduces the formation of the poor oxide (e.g., Al[OH]
3
) when the device is operated in wet, high-temperature environments. Within this description, there can be different degrees of passivation for devices that have been subjected to wet, high-temperature operating life (WHTOL) testing based on the amount of device degradation after operating for a fixed time. For example, a light emitting diode (LED) may be considered fully-passivated if the emitted light output power (LOP) has degraded less than 20% after 2000 hours of WHTOL operation. An LED is considered partially-passivated if the LOP has degraded less than 50% after 2000 hours of WHTOL operation. Thus, the term “passivated” describes devices from partially to fully passivated. Herein, the conditions of a WHTOL test are under 20 mA loading (i.e., forward bias in an LED) in an atmosphere of 85% relative humidity and a temperature of 85° C.
A method for forming high quality native oxides through the use of a water vapor environment at elevated temperatures is described in Holonyak '360 and is applicable herein. A wide range of temperatures is described between 375° C. to 1000° C. to grow native oxides in Al-bearing materials. In Holonyak '039 it was specified that the most critical areas of a semiconductor device to passivate were those in which the majority of the light generated by the light emitting diode (LED) are transmitted. This is based on the belief that corrosion was accelerated by photon interactions. Holonyak '039 also specifies the need to control the temperature and time of the oxide growth period so the thickness of the native oxide growth is within a particular thickness range. Specifically, the native oxide film must be thicker than 0.1 um to avoid pinholes or cracks in the film, but thinner than 7.0 um which can cause cracks in the film due to internal stress. The cracks can prevent complete passivation and result in light output loss during WHTOL tests. In Holonyak '039 it also stated that that the water vapor oxidation temperature range should be 375° C. to 550° C., preferably from 450° C. to 550° C., and the most preferable oxidation time is 0.25 hour to 2 hours.
In light emitting devices it is often desirable to incorporate wide band gap, high Al content layers for improved carrier confinement, carrier injection, wave guiding properties, etc. For example, it is known that the emission efficiencies of red-emitting aluminum gallium arsenide (AlGaAs) LEDs can be improved by increasing the Al-mole fraction, x, of the high-composition Al
X
Ga
(1−x)
As confining layers immediately adjoining the active layer. However, the destructive oxide degradation problems have limited the content of these Al
x
Ga
(1−x)
As layers to the range where the Al mole-fraction, x, is less than 0.75. The mole fraction, x, indicates the amount of Al in the layer and is defined as the fractional composition of Al to the Group III element in the layer.
The prior art has shown the performance of Al-bearing semiconductor devices can be greatly improved through the use of native oxide passivation. Although many issues have been addressed, there is still no viable method for using this technique in high volume manufacturing. To successfully implement this technology, it is critical to have processing techniques that can be used to passivate the device areas that have the greatest impact on the device performance and reliability.
DISCLOSURE OF THE INVENTION
It has been discovered that the most critical areas for passivation are the highest aluminum (Al)-content exposed layers of the device. For Al-bearing substrate AlGaAs LEDs, the confining layers adjoining the active layer possess the highest Al content. Failure analysis of non-passivated WHTOL-aged Al-bearing substrate AlGaAs LEDs indicates that corrosion occurs the fastest at the exposed surfaces of the high-Al content confining layers. By placing a high-quality native oxide at the exposed surfaces of the high-composition Al-content confining layers which protect from the formation of the ‘poor’ oxide, it is possible for LEDs to retain essentially their same light output after 2,000 hours of WHTOL testing.
Although passivating the highest exposed Al-content layers improves the WHTOL degradation, partial passivation results in only partial WHTOL protection. It is desirable and optimal to passivate all or the majority of the exposed Al-bearing layers in the light-emitting device structure. Such full passivation results in the optimal WHTOL performance for Al-bearing substrate AlGaAs LEDs. However, a consequence of full-wafer processing is that it is very difficult (or nearly impossible) to completely expose all edges of the devices (by singulating them) prior to oxidation. The present invention provides a method of exposing the majority of the edges, especially those with the highest Al-bearing layers, leaving the remaining layers intact and connected. This structure can be realized by etching mesas (using wet chemical and/or dry plasma processes), sawing partially through the wafer, or a combination of both. Such processing facilitates the exposure of the layers most prone to degradation for oxidation-passivation while allowing full-wafer processing. The oxidation can occur prior to or after the deposition of the top and/or bottom metallization layers or contacts. Areas that are left connected should be of the lowest Al-content (or be Al-free). If the connected layers do contain Al, the surface areas of these layers should be kept to a minimum compared to the remainder of the device area. In addition, it is preferable that the non-oxidized exposed surfaces of the connected layers pass a minimum amount of light after singulation under device operation to minimize any photon-assisted degradation.
In an Al-free substrate AlGaAs LED, a thick GaAs substrate can be employed as a connected carrier layer, allowing all of the Al-bearing layers to be exposed to oxidation-passivation. Again, depending on the oxidation conditions, the lower Al-content active layer may or may not be passivated.
For Al-free and Al-bearing substrate AlGaAs LEDs, it is necessary to oxidize the LEDs from 500 to 625° C. for 1 to 60 minutes to ensure passivation. It has been determined that 600° C. for 5 minutes gives the best results for standard Al-bearing substrate AlGaAs LEDs and 550° C. for 6 minutes is optimal for Al-free substrate AlGaAs devices. Different Al contents need different temperatures to create a robust enough oxide for moisture resistance. The thickness of the oxide can also play a part. If an oxide is too thin, it can have pinholes, but
Khare Reena
Kish Fred A.
Jackson, Jr. Jerome
LumiLeds Lighting U.S., LLC
Ogonowsky Brian D.
Schmidt Mark E.
Skjerven Morrill & MacPherson LLP
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