GB2306640A - Photoluminscent determination of impurity concentrations in a semiconductor - Google Patents
Photoluminscent determination of impurity concentrations in a semiconductor Download PDFInfo
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- GB2306640A GB2306640A GB9521226A GB9521226A GB2306640A GB 2306640 A GB2306640 A GB 2306640A GB 9521226 A GB9521226 A GB 9521226A GB 9521226 A GB9521226 A GB 9521226A GB 2306640 A GB2306640 A GB 2306640A
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- impurity concentration
- quantum well
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- G01N21/62—Systems in which the material investigated is excited whereby it emits light or causes a change in wavelength of the incident light
- G01N21/63—Systems in which the material investigated is excited whereby it emits light or causes a change in wavelength of the incident light optically excited
- G01N21/64—Fluorescence; Phosphorescence
- G01N21/6489—Photoluminescence of semiconductors
Abstract
The impurity concentration in a sample of semiconductor material is investigated by using it to form at least one barrier layer in a heterostructure in which a quantum well layer is sandwiched between barrier layers of a higher bandgap semiconductor material compared to the bandgap of the semiconductor material of the quantum well layer. The material is subjected to photoluminescence spectroscopy and the intensity of a lower energy peak in a doublet is determined. This is compared with the equivalent peak height from other samples to determine relative impurity concentrations. To obtain an absolute value of impurity concentration, the peak intensity is compared with that obtained from a calibration sample of known impurity level.
Description
METHOD OF CHARACIERISII5G A SEMICONDUCTOR MATERIAL The present invention relates to a method of characterising a semiconductor material, and in particular, a method of determining the absolute or relative impurity concentration in that material.
When a heterostructure is formed, comprising for example, a quantum well layer of GaAs sandwiched between AIxGai.xAs barrier layers, when the latter are formed by Molecular Beam Epitaxy (MBE),
Metal-Organic Chemical Vapour Disposition (MOCVD) or Metal-Organic
Vapour Phase Epitaxy (MOVPE), the AlxGal,xAs usually incorporates a much larger density of impurities than the GaAs layer.
The impurity concentration of the barriers is of concern because the barrier impurities can limit the performance of quantum well and other similar types of heterostructure devices. For instance, they can reduce the mobility of GaAs/AlxGal,xAs high electron mobility transistors (HEMTs) under certain conditions, since they cause scattering of the electrons in the
GaAs layer. Barrier impurities also impair the performance of p-i-n multiple quantum well optical modulators and other multiple quantum well electro optic devices working on the quantum confined Stark effect In this case, the impurities produce a space charge within the barrier material which causes the field to change across the intrinsic region.Since each well within the intrinsic region will then experience a different electric field, this can severely broaden the optical response of the multiple quantum well. However, there are other examples where the impurity density is important.
The shallow impurities in the barrier layers create free carriers in the quantum well. ("Shallow" means that the impurity states have an energy close to the conduction band edge.) Hence if the barriers contain shallow donor atoms these will create excess electrons in the well. This is because the conduction band in the well layer lies lower in energy than the energy of the shallow donors in the barrier. On the other hand, if the barriers contain more shallow acceptors, these will create excess holes in the well.
Again, the reason for this is that the energy of the valence band in the well is higher than the energy of acceptor ions in the barriers, as can be seen in
Figure 1 of the accompanying drawings.
It has now been discovered that the impurity concentration in such barrier layers can be determined by subjecting them to photoluminescence spectroscopy. Thus, the present invention provides a method of investigating the impurity concentration in a barrier layer, which barrier layer is formed of a sample semiconductor material, said barrier layer being incorporated in a heterostructure which also comprises a quantum well layer of semiconductor material having a first bandgap, disposed between first and second barrier layers of semiconductor material of higher bandgap than said first bandgap, one of which barrier layers is said barrier layer of sample semiconductor material, the method comprising subjecting said barrier layer of sample semiconductor material to photoluminescence spectroscopy.
However, the discovery that photoluminescence spectroscopy of a barrier layer in such a quantum well structure also leads to the realisation that the impurity concentration in a sample of semiconductor material intended for a different type of application can still be investigated by first incorporating some of it in the aforementioned kind of quantum well structure.Thus, the present invention also provides a method of investigating the impurity concentration in a sample of semiconductor material, the method comprising forming a heterostructure comprising a quantum well layer of semiconductor material having a lower bandgap than that of said sample semiconductor material, said quantum well layer being disposed in said heterostructure between first and second barrier layers of higher bandgap semiconductor material, at least one of said first and second barrier layers being formed of said sample semiconductor material, and subjecting the at least one barrier layer formed of said sample semiconductor material to photoluminescence spectroscopy.
Clearly, it is most convenient to make both of the barrier layers from the same sample semiconductor material.
The method of the present invention can be used to determine the relative concentrations of impurities between successive samples or an absolute measure of the impurity concentration in a given sample. In either case, the impurity concentration is derived from the intensity of a certain peak in the photoluminescence (PL) spectrum. The method of the present invention is sensitive to impurity concentration of order 1014and3 and higher. This compares well to other techniques for detaining impurity concentrations such as secondary-ion mass sperometry, local vibrational mode spectroscopy and capacitance-voltage profiling. It can also be applied to many quantum well devices in a non-destructive manner.
The PL spectra of quantum wells have been found to show a doublet structure at suitably low temperatures, with a splitting of about 1-2 meV.
The size of the splitting depends on the well width and the system being studied. The lower energy component of this doublet is caused by the barrier impurities and its intensity is proportional to the barrier impurity concentration. The intensity of this lower energy peak can therefore be used either, to compare the relative barrier impurity concentrations of different samples, or to estimate the absolute concentration in a sample by comparison to a reference with known impurity concentration.
Another method of determining absolute impurity concentration is by comparing the intensity of the lower energy peak in the doublet, relative to the intensity of the higher energy peak in that doublet.
Without wishing to be bound by any particular theory or explanation, the inventors have conjectured that the lower energy peak is due to the so-called positively charged exciton (X+) formed when the recombining electron and hole bind to an excess hole in the quantum well.
These holes are present in the well due to ionisation of the barrier acceptors. For MBE grown AlxGal,xAs, the barrier impurities are usually mostly p-type (i.e. acceptors), which means they take an electron to leave a hole in the well. The background impurities of MOCVD- and MOVPEgrown AlxGai.xAs are usually also mostly acceptors. More acceptors in the barriers means more holes in the well and a stronger PL peak due to
X+.
Although, for the MBE grown AlxGal xAs, the impurities are mostly acceptors, the method of the present invention can also be applied to material where the majority of the barrier impurities are donors. This is certainly the case for AlxGal-xAs grown by r iquid-Phase-Epitaxy (LPE), for instance. In that case, the donors in the barriers will create an excess of electrons in the well. These will bind to the recombining electron and hole to form a negatively-charged exciton. The negatively-charged exciton also produces a PL peak below that of the free exciton. The intensity of this low energy peak will be proportional to the concentration of donors in the barriers.
Photoluminescence spectroscopy is a very well known technique.
The following experimental conditions are generally speaking, necessary for characterisation of the samples by this method (1) The thermal energy (k T, where k is Boltnnann's constant and T is the temperature in Kelvin) should be less than the second carrier binding energy of the charged exciton. This is described in further detail hereinbelow. For GaAs/AlxGal,xAs quantum wells this means that the temperature should be below 10 K Such temperatures are routinely available with liquid He cryostats.
(2) The laser energy (hu) should be larger than the bandgap of the well material but below that of the barriers, so that photons are absorbed directly in the well. For GaAs/A10.33Gao 67As quantum wells, the laser energy should be between about 1.6 and 1.8 eV.
(3) The laser power density should not cause heating of the sample and be less than 1 W/cm~2 (for CW excitation).
(4) Samples to be compared should be measured under identical conditions.
The present invention is not limited to GaAs/AlxGal,xAs systems.
Other quantum well heterostructures may also be used, for example
GaAs/A1As, InxGa1-xAs/AlyGa1-xAs/InP,
Gaxln1-P/(AlyGa1-y)zIn1-zP,Zn1 xCdxSe/ZnSe, ZnSe/Zn1-xcMgxSySe1 y, Znl ,CdcSe/ZnSz y, CdTe/Cdl cZnxTe, GaN/AlN, GaNiAixGa 1 .xN, InxGa 1 .xN!GaN, InGa1(-xN/AlyGal yN.
However, the technique is most useful for system where the barrier material incorporates more impurities than the well. The basic requirements for sample temperature and laser energy will be different than those mentioned above for GaAs/AlxGa1-xAs. However, appropriate experimental conditions can be determined for each system from the conditions (1) to (4) outlined above. For example, for CdTefCd1xZnxTe quantum wells, the larger charged exciton binding energy means that the temperature should be less than 30 K The maximum laser energy that could be used depends on the Zn model fraction in the barriers, but could be about 1.7 eV.
As mentioned above, the method of the present invention can be used to study quantum well devices directly, or alternatively, a suitable quantum well structure can be grown to assess the quality of the material in non-quantum well devices. The quantum well width should be sufficiently narrow to squeeze the exciton along the growth direction. On the other hand, it should not be so narrow as to cause severe inhomogenous broadening of the excitonic lines. For GaAs/AlxGal,xAs quantum wells this corresponds to a well width between roughly 10 and 30nm.
In a variant of this technique one could focus the laser light to a small spot (of about 0.5 micron diameter) on the sample surface using a microscope objective. With such a small spot, the variation of the barrier impurity concentration across the wafer can be mapped by scanning, eg with roughly 0.5 micron spatial resolution. Higher resolution, of about 0.05 microns, can be achieved with a near field microscope which illuminates the sample using an optical fibre with a fine tip positioned close to the sample surface.
The present invention will now be explained in more detail by way of the following non-limiting description of example methods for carryingout the present invention and with reference to the accompanying drawings, in which:
Figure 1 shows an energy band diagram for explaining how shallow acceptors in barrier layers create excess holes in a quantum well layer;
Figure 2 shows the layer structure of samples used for carrying-out an example of a method according to the present invention;
Figure 3 shows a comparison of PL spectra obtained from Samples 1 and 3; and
Figure 4 shows PL spectra (plotted on a logarithmic scale) recorded for Sample 2 at different temperatures.
EXAMPLES
Four wafers with an identical layer structure were investigated, each consisting of four GaAs quantum wells separated by A10 33Gao.67As barriers grown by Molecular Beam Epitaxy (MBE) on an undoped GaAs (100) oriented epiready substrate 1. The full layer structure is shown in
Figure 2.It consisted of a 1 pm GaAs buffer layer 3 on which the following additional layers were formed in this order: a 300A A10 33Gao.67As first barrier layer 5; a 300A GaAs first quantum well layer 7; a 300A Al0.33Ga0.67As second barrier layer 9; a 200A GaAs second quantum well layer 11; a 300A A1o.33Gao.67A5 third barrier layer 13; a 165A GaAs third quantum well layer 15; a 300A A10 33Gao.67As fourth barrier layer 17; a l4OA GaAs fourth quantum well layer 19; a 30ûoA A10 33Gao.67As as fifth (and final, upper) barrier layer 21; and finally, a 100 GaAs capping layer 23.
The four samples were nominally identical except for the substrate temperature during growth, which was 630 C for Sample 1, 6PC for
Sample 2, 630 C for Sample 3, and 610 C for Sample 4. Sample 1 was grown two months prior to Samples 2-4, at a time when the growth chamber had a higher background impurity concentration.
An indication of the background impurity level is given by the mobilities of ungated High Electron Mobility Transistors (HEMTs) grown in the same MBE chamber. A short time before Sample 1 was grown, a
GaAs/Al0.33Ga0.67As HEMT with a 200 A spacer layer was grown with a mobility of 1.0x106 cm2V-ls-l after illumination. On the other hand, an identical HEMT structure grown around the time of Samples 2-4 showed a higher mobility of 1.8x106 cm2V-ls-l after illumination, suggesting the background impurity concentration is significantly reduced. Generally, the
HEMT mobility improves over the several month duration of a growth run as the background impurity concentration in the MBE chamber decreases.
Although none of the layers in the structure were intentionally doped, MBE grown AlO.33Ga0.67As is susceptible to the incorporation of significant levels of p-type dopants, with carbon generally thought to be the principle acceptor species.
The PL spectra were obtained by placing the samples in a variable temperature He cryostat. The sample was excited with a tuneable Ti
Sapphire laser incident normal to the sample surface. The PL emitted normal to the sample surface was collected and dispersed by a single grating spectrometer of 0.64 m focal length and detected by a liquid nitrogen cooled CCD.
Figure 3 plots PL spectra measured on Samples 1 and 3. The excitation energy (EL) and density (PL) were 1.72eV and 42 mWcm-2, respectively, while the sample temperature was 2.or. Each of the four quantum wells in each sample shows a strong peak due to the recombination of the free Is exciton (X) formed between an electron and a heavy hole in their lowest confined subbands, as marked for Sample 3 in
Fig. 3. Each quantum well also displays a weaker PL peak around 0.97 1.35 meV below the free exciton, due to the positively charged exciton (X+).
The splittings of the two PL peaks of the doublet emanating from each quantum well, which are listed in Table 1, increase with decreasing well width. This splitting can be crudely equated to the binding energy of the second hole in X+ to a neutral exciton.
TABLE 1:
The Splitting of the PL Doublet Measured for Each of the Ouantum Wells
in each of the Four Samples
Binding Energy (meV) Well width (A) Sample 1 Sample 2 Sample 3 Sample 4 Average 140 1.35 1.37 1.36 1.33 1.33j0.O1 165 1.12 1.12 1.11 1.12 1.12j().01 200 1.10 1.06 1.10 1.06 1.08+0.02 300 0.98 0.95 0.97 0.96 0.97+0.01 Notice that the lower energy components due to (X+) of the doublets are more prominent for Sample 1 than for Sample 3, despite the two samples having identical layer specification and being grown under the same conditions. This is indicative that the background impurity concentration of the barriers is higher in Sample 1 than Sample 3.This correlates with the mobility of a HEMT structure grown around the time of
Sample 1 being lower than an identical HEMT grown around the time of
Sample 3. The higher background impurity concentration in Sample 1 may also explain why its PL lines are slightly broader than for Sample 3. The difference in the excitonic energies of Samples 1 and 3 suggests a variation in the growth rate between the samples. Samples 2 and 4 showed very similar PL spectra to Sample 3, consistent with them having been grown around the same time as Sample 3.
Thus, for a given sample, the relative or absolute impurity level (for a given uniform well thickness) for the AlxGal xAs barrier layer material is determined by comparing the integrated intensity of the lower energy peak in the relevant doublet with the corresponding peak obtained under the same conditions, using either another example or a calibration sample of known impurity concentration, as appropriate.
The requirement that the sample temperature be sufficiently low so that the thermal energy is smaller that the second-hole binding energy of
X+ is highlighted by Figure 4. Figure 4 shows plots on a log scale of PL recorded on Sample 2 at different sample temperatures. At 2 K the lower energy component of each of the doublets (due to X+) is relatively strong.
However, as the temperature is increased the lower energy peak weakens sharply, while the higher energy component strengthens slightly, so that only the latter is readily discernible at 15 K. There is also a strengthening with temperature of the light-hole exciton line of the 300 A quantum well, in addition to the 2s state of the heavy-hole exciton. The latter can just be discerned at 15 K near 1.5227 eV. A very similar temperature dependence was observed for the PL doublets of the other samples.
The weakening of the lower energy peak due to X+ with increasing temperature is explained by its thermal dissociation into a neutral exciton and a free hole. This occurs when the thermal energy is comparable to the second hole binding energy of X+. A binding energy of 1 meV, equal to the doublet splitting for the 300 A quantum well, implies a disassociation temperature of 12 K in good agreement with the dependence shown in
Figure 4.
It is important to use a laser energy which is less than the bandgap of the AlxGal.xAs barriers. For excitation at an energy above the barrier bandgap, holes photo-excited in the uppermost barrier layer are swept by the internal electric field of the structure into the well. Hence under this condition the strength of the X+ PL peak will no longer be related to the acceptor concentration of the barriers.
In the light of this disclosure, modifications of the described examples, as well as other examples, all within the scope of the present invention as defined by the appended claims, will now become apparent to persons skilled in the art.
Claims (15)
1. A method of investigating the impurity concentration in a sample of semiconductor material, the method comprising forming a heterostructure comprising a quantum well layer of semiconductor material having a lower bandgap than that of said sample semiconductor material, said quantum well layer being disposed in said heterostructure between first and second barrier layers of higher bandgap semiconductor material, at least one of said first and second barrier layers being formed of said sample semiconductor material, and subjecting the at least one barrier layer formed of said sample semiconductor material to photoluminescence spectroscopy.
2. A method of investigating the impurity concentration in a barrier layer, which barrier layer is formed of a sample semiconductor material, said barrier layer being incorporated in a heterostructure which also comprises a quantum well layer of semiconductor material having a first bandgap, disposed between first and second barrier layers of semiconductor material of higher bandgap than said first bandgap, one of which barrier layers is said barrier layer of sample semiconductor material, the method comprising subjecting said barrier layer of sample semiconductor material to photoluminescence spectroscopy.
3. A method according to claim 1 or claim 2, wherein both of said first and second barrier layers are formed of said sample semiconductor material.
4. A method according to any preceding claim, wherein the step of perfonning photoluminescence spectroscopy comprises determining the intensity of the lower energy peak of two peaks in a doublet in the photoluminescence spectrum.
5. A method according to claim 4, wherein the lower peak intensity is compared with the equivalent lower energy peak intensity in another sample to determine the relative impurity concentrations between the sample semiconductor materials in each sample.
6. A method according to claim 4, wherein the value of the lower energy peak intensity is used to calculate the absolute value of the impurity concentration in the sample semiconductor material.
7. A method according to claim 6, wherein the absolute value of the impurity concentration is calculated by comparing the measured lower peak intensity with that obtained under the same conditions, using a calibration sample of known impurity concentration.
8. A method according to claim 4, wherein the value of the lower energy peak intensity is compared with the corresponding value of the higher energy peak in the doublet.
9. A method according to any preceding claim, wherein the photoluminescence spectroscopy is formed by scanning with a laser beam across a sample wafer to map the impurity concentration across the wafer.
10. A method according to any preceding claim, wherein the photoluminescence spectroscopy is performed with a sample temperature corresponding to a thermal energy less than the second carrier binding energy of the charged exciton.
11. A method according to any preceding claim, wherein the photoluminescence spectroscopy is performed by irradiating the sample with an energy (hv) larger than the bandgap of the semiconductor material of the quantum well layer but lower than that of either of the semiconductor materials forming the first and second barrier layers.
12. A method according to any preceding claim, wherein the photoluminescence spectroscopy is performed by irradiating with an incident power density of less than 1 W/cm~2 and less than that required to heat the sample.
13. A method according to any preceding claim, wherein the quantum well layer is formed of GaAs and the barrier layers are formed of AlxGa l-xAs.
14. A method according to claim 13, wherein the quantum well layer has a thickness of from lOnin to 30nm
15. A method of investigating the impurity concentration in a sample of semiconductor material, the method being substantially as hereinbefore described with reference to any of the examples.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB9521226A GB2306640B (en) | 1995-10-17 | 1995-10-17 | Method of characterising a semiconductor material |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB9521226A GB2306640B (en) | 1995-10-17 | 1995-10-17 | Method of characterising a semiconductor material |
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| Publication Number | Publication Date |
|---|---|
| GB9521226D0 GB9521226D0 (en) | 1995-12-20 |
| GB2306640A true GB2306640A (en) | 1997-05-07 |
| GB2306640B GB2306640B (en) | 1998-01-14 |
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| Application Number | Title | Priority Date | Filing Date |
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| GB9521226A Expired - Fee Related GB2306640B (en) | 1995-10-17 | 1995-10-17 | Method of characterising a semiconductor material |
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Cited By (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP1731897A3 (en) * | 2005-06-06 | 2009-07-22 | Sumitomo Electric Industries, Ltd. | Damage evaluation method and production method of compound semiconductor member |
| US8064054B2 (en) | 2005-10-11 | 2011-11-22 | Bt Imaging Pty Ltd | Method and system for inspecting indirect bandgap semiconductor structure |
| US20120181452A1 (en) * | 2009-07-20 | 2012-07-19 | Bt Imaging Pty Ltd. | Separation of doping density and minority carrier lifetime in photoluminescence measurements on semiconductor materials |
| EP2952883A1 (en) * | 2014-05-23 | 2015-12-09 | GLobalWafers Japan Co., Ltd. | Calibration curve formation method, impurity concentration measurement method, and semiconductor wafer manufacturing method |
Citations (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4492871A (en) * | 1981-10-07 | 1985-01-08 | Agency Of Industrial Science & Technology | Method for determining impurities in epitaxial silicon crystals |
-
1995
- 1995-10-17 GB GB9521226A patent/GB2306640B/en not_active Expired - Fee Related
Patent Citations (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4492871A (en) * | 1981-10-07 | 1985-01-08 | Agency Of Industrial Science & Technology | Method for determining impurities in epitaxial silicon crystals |
Cited By (8)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP1731897A3 (en) * | 2005-06-06 | 2009-07-22 | Sumitomo Electric Industries, Ltd. | Damage evaluation method and production method of compound semiconductor member |
| US8177911B2 (en) | 2005-06-06 | 2012-05-15 | Sumitomo Electric Industries, Ltd. | Damage evaluation method of compound semiconductor member, production method of compound semiconductor member, gallium nitride compound semiconductor member and gallium nitride compound semiconductor membrane |
| US8064054B2 (en) | 2005-10-11 | 2011-11-22 | Bt Imaging Pty Ltd | Method and system for inspecting indirect bandgap semiconductor structure |
| US20120181452A1 (en) * | 2009-07-20 | 2012-07-19 | Bt Imaging Pty Ltd. | Separation of doping density and minority carrier lifetime in photoluminescence measurements on semiconductor materials |
| US8742372B2 (en) * | 2009-07-20 | 2014-06-03 | Bt Imaging Pty Ltd | Separation of doping density and minority carrier lifetime in photoluminescence measurements on semiconductor materials |
| US9157863B2 (en) | 2009-07-20 | 2015-10-13 | Bt Imaging Pty Ltd. | Separation of doping density and minority carrier lifetime in photoluminescence measurements on semiconductor materials |
| EP2952883A1 (en) * | 2014-05-23 | 2015-12-09 | GLobalWafers Japan Co., Ltd. | Calibration curve formation method, impurity concentration measurement method, and semiconductor wafer manufacturing method |
| US9541452B2 (en) | 2014-05-23 | 2017-01-10 | Globalwafers Japan Co., Ltd. | Calibration curve formation method, impurity concentration measurement method, and semiconductor wafer manufacturing method |
Also Published As
| Publication number | Publication date |
|---|---|
| GB9521226D0 (en) | 1995-12-20 |
| GB2306640B (en) | 1998-01-14 |
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